diff --git a/tools/neuron-sandbox/STACK.md b/tools/neuron-sandbox/STACK.md
new file mode 100644
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+++ b/tools/neuron-sandbox/STACK.md
@@ -0,0 +1,120 @@
+# sandbox — the Neuron STACK sandbox
+
+**Work on a whole stack at once, not one repo at a time.** `sandbox` assembles every
+constituent repo of a named stack into **one combined worktree workspace**, wired so
+they build and run **together**, on an isolated clean base — then tears it all down
+cleanly. The live soul/engram (`:7770` / `:8742`) are never touched.
+
+It is the multi-repo sibling of [`nsbx`](./README.md): where `nsbx dev` stands up
+**one** repo's worktree + an isolated engram, `sandbox` stands up **every** repo of a
+stack as sibling git worktrees under a single workspace.
+
+```bash
+export PATH="$PWD:$PATH"            # or symlink `sandbox` onto your PATH
+
+sandbox neuron-stack tim           # el + neuron soul + NeuronUI, assembled together
+cd ~/Development/neuron-technologies/stack-worktrees/neuron-stack-tim
+source .stack-env                  # EL_REPO + PATH now point at the SANDBOX el
+./build.sh                         # engram compiles, soul compiles, UI present & buildable
+sandbox down neuron-stack tim      # remove every worktree; live untouched
+```
+
+## The two profiles
+
+### `el-stack` — the whole EL kit
+The compiler + language + framework + tooling are **all one repo** (`foundation/el`:
+`lang/` = elc/elb + runtime, `engram/src/server.el`, `elp/` = NLG, `ui/` = the **el-ui
+framework**, plus `ql`, `ide`, `epm`, `arbor`, `tools`). Its downstream SDK consumers
+come along so a change to `elc` can be proven end-to-end across the kit.
+
+| repo | required | role |
+|------|----------|------|
+| `foundation/el`     | ✓ | elc + elb compiler, el_runtime, engram source, **el-ui framework**, elp/ql/ide/epm tooling |
+| `engram-language`   |   | language-faculty reference POC (Python) — being ported into `el/elp` |
+| `foundation/forge`  |   | downstream SDK consumer — `make build` |
+| `foundation/dharma` |   | downstream SDK consumer — CGI provenance registry |
+
+`build.sh` proves it: `elc` compiles a real stack source and `cc` links it against the
+runtime into a native binary (elc + runtime build together), and — if present — `forge`
+builds against the freshly-assembled SDK.
+
+### `neuron-stack` — the full product
+Substrate + soul + UI. **Engram is not a separate repo** — its source lives inside
+`foundation/el`.
+
+| repo | required | role |
+|------|----------|------|
+| `foundation/el`     | ✓ | substrate: elc + el_runtime + engram source + the `elp` NLG the soul imports |
+| `neuron`            | ✓ | the soul (`:7770`) + engram build; `soul.el` imports `../foundation/el/elp/src/elp.el` |
+| `products/NeuronUI` | ✓ | the app/UI (Kotlin/Compose desktop client; bundles the soul binary) |
+| `products/web`      |   | marketing site + interactive soul-demo |
+
+`build.sh` proves it: **engram** builds (`elc engram/src/server.el` → `cc … el_runtime.c`
+→ native binary), the **soul** compiles with its cross-repo `../foundation/el` import
+resolving to the *sandbox* el, and the **UI** is present with its build entry.
+
+## Why it works — mirrored-layout wiring
+
+The repos reference each other by **relative sibling paths** (e.g. the soul imports
+`../foundation/el/elp/src/elp.el`). So `sandbox` lays every worktree out at its **natural
+relative path** inside the workspace:
+
+```
+stack-worktrees/neuron-stack-tim/
+├── foundation/el/          ← worktree of foundation/el   (the sandbox el)
+├── neuron/                 ← worktree of neuron
+└── products/NeuronUI/      ← worktree of products/NeuronUI
+```
+
+From `neuron/`, `../foundation/el` resolves to `…/neuron-stack-tim/foundation/el` — the
+**sandbox** copy, never the live tree. No symlinks, no path rewriting: the layout *is*
+the wiring. `.stack-env` additionally pins `EL_REPO` and prepends the sandbox `elc`/`elb`
+to `PATH`.
+
+## Commands
+
+| command | does |
+|---------|------|
+| `sandbox el-stack <name> [--minimal]`     | assemble the EL kit (`--minimal` = required repos only) |
+| `sandbox neuron-stack <name> [--minimal]` | assemble the full product |
+| `sandbox build <profile> <name>`          | run the workspace's combined `build.sh` |
+| `sandbox status <profile> <name>`         | per-repo head + clean/dirty |
+| `sandbox list`                            | list assembled workspaces |
+| `sandbox down <profile> <name> [--delete-branch]` | remove every worktree + drop the workspace (branch kept unless `--delete-branch`) |
+
+Flags: `--minimal` (required repos only), `--branch B` (branch name; default
+`sandbox/<profile>-<name>`), `--base REF` (fork point; default each repo's committed
+HEAD).
+
+## Rails (always)
+
+- **Clean base** — worktrees fork off each repo's **committed HEAD**; the dirty state of
+  the live checkout is deliberately *not* carried in.
+- **Persistent** — the workspace lives under `NSBX_STACK_ROOT` (default
+  `~/Development/neuron-technologies/stack-worktrees`), **never `/tmp`** (ablated on
+  compaction).
+- **Never touches live** — `sandbox` only does `git worktree` + offline `cc`. It never
+  binds `:8742`/`:7770`, never `launchctl`, never `pkill`. Bringing up an **isolated
+  engram** is delegated, opt-in, to `nsbx` (which guards the live store and refuses the
+  live ports).
+- **Idempotent & safe** — refuses to clobber an existing workspace; a failed assembly
+  rolls back its partial worktrees; teardown removes worktrees through their origin repo
+  and prunes.
+- **Own-the-core** — pure bash + `git worktree`. No new dependencies.
+
+## Env knobs
+
+`NSBX_STACK_ROOT` (workspace root), `NEURON_DEV_ROOT` (the dir holding all the peer
+repos, default `~/Development/neuron-technologies`).
+
+## Isolated engram for `neuron-stack` (opt-in)
+
+`sandbox` gets the code building together; to run the soul against an **isolated** engram
+(never live), delegate to `nsbx` from inside the workspace:
+
+```bash
+source .stack-env
+nsbx create $STACK_NAME --source "$EL_REPO"    # clone live store onto a non-default port
+nsbx up     $STACK_NAME
+nsbx status $STACK_NAME                          # prints the isolated engram URL
+```
diff --git a/tools/neuron-sandbox/sandbox b/tools/neuron-sandbox/sandbox
new file mode 100755
index 0000000..dd9dfce
--- /dev/null
+++ b/tools/neuron-sandbox/sandbox
@@ -0,0 +1,505 @@
+#!/usr/bin/env bash
+# sandbox — the Neuron STACK sandbox: assemble a WHOLE stack of repos into ONE
+# combined worktree workspace, wired so they build/run TOGETHER, on an isolated
+# clean base — so an agent (or Will) can work on the full stack at once instead of
+# one repo at a time.
+#
+# It is the multi-repo generalisation of `nsbx` (this same directory): where
+# `nsbx dev` stands up ONE repo's worktree + an isolated engram, `sandbox` stands
+# up EVERY constituent repo of a named stack as sibling git worktrees under a
+# single workspace, mirroring their on-disk relative layout so the cross-repo
+# `../foundation/el` imports resolve to the SANDBOX copy — never the live tree.
+#
+#   sandbox el-stack     <name>      # elc compiler + EL language + framework + tooling (+ consumers)
+#   sandbox neuron-stack <name>      # runtime/soul + engram + app/UI (the full product)
+#   sandbox list                     # list assembled stack workspaces
+#   sandbox status <profile> <name>  # inspect one
+#   sandbox build  <profile> <name>  # run the workspace's combined build.sh
+#   sandbox down   <profile> <name>  # tear down: remove every worktree, drop the workspace
+#
+# RAILS (always):
+#   * worktrees fork off each repo's COMMITTED HEAD -> a clean, reproducible base
+#     (the dirty state of the live checkout is deliberately NOT carried in).
+#   * the workspace lives at a PERSISTENT path (never /tmp — ablated on compaction).
+#   * NEVER touches the live soul/engram (:7770 / :8742). It only creates git
+#     worktrees + a build script; bringing up an isolated engram is delegated,
+#     opt-in, to `nsbx` (which already guards the live store & ports).
+#   * idempotent & safe: refuses to clobber an existing workspace; teardown removes
+#     worktrees through their origin repo and prunes — branches are kept by default.
+#   * own-the-core: pure bash + git worktree. No new dependencies.
+set -uo pipefail
+
+# ---------------------------------------------------------------- constants ----
+# Root that holds all the peer repos (neuron, foundation/el, products/*, ...).
+DEV_ROOT="${NEURON_DEV_ROOT:-$HOME/Development/neuron-technologies}"
+# Where assembled stack workspaces live (persistent; sibling to el-worktrees/).
+STACK_ROOT="${NSBX_STACK_ROOT:-$DEV_ROOT/stack-worktrees}"
+EL_REPO_REL="foundation/el"
+LIVE_ENGRAM_PORT=8742          # live engram  — sandbox must never bind it
+LIVE_SOUL_PORT=7770            # live soul    — sandbox must never bind it
+# nsbx (single-repo isolated-engram tool) lives next to this script.
+NSBX="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)/nsbx"
+
+C_RED=$'\033[31m'; C_GRN=$'\033[32m'; C_YEL=$'\033[33m'; C_CYN=$'\033[36m'; C_DIM=$'\033[2m'; C_BLD=$'\033[1m'; C_0=$'\033[0m'
+
+# ---------------------------------------------------------------- helpers ------
+die(){ printf '%serror:%s %s\n' "$C_RED" "$C_0" "$*" >&2; exit 1; }
+log(){ printf '%s==>%s %s\n' "$C_BLD" "$C_0" "$*" >&2; }
+info(){ printf '    %s\n' "$*" >&2; }
+ok(){   printf '    %s%s%s\n' "$C_GRN" "$*" "$C_0" >&2; }
+warn(){ printf '    %s%s%s\n' "$C_YEL" "$*" "$C_0" >&2; }
+need(){ command -v "$1" >/dev/null 2>&1 || die "missing dependency: $1"; }
+
+# ---------------------------------------------------------------- profiles -----
+# profile_repos <profile> : emit one line per constituent repo:
+#   <relpath-under-DEV_ROOT> | <required|optional> | <role>
+# The relpath is preserved INSIDE the workspace, so all cross-repo `../foundation/el`
+# references resolve to the sandbox copy automatically (mirrored-layout wiring).
+profile_repos(){
+  case "$1" in
+    el-stack)
+      # The compiler+language+framework+tooling are all ONE repo (foundation/el).
+      # Its downstream SDK consumers (forge, dharma) + the language-faculty POC come
+      # along so a change to elc can be proven end-to-end across the kit.
+      cat <<'EOF'
+foundation/el      | required | elc + elb compiler, el_runtime, engram source, el-ui framework, elp/ql/ide/epm tooling
+engram-language    | optional | language-faculty reference POC (Python) — ported into el/elp
+foundation/forge   | optional | downstream SDK consumer — `make build` (imprint forge CLI)
+foundation/dharma  | optional | downstream SDK consumer — CGI provenance registry
+EOF
+      ;;
+    neuron-stack)
+      # The full product: substrate (el) + soul + UI. Engram is NOT a separate repo
+      # (its source lives in foundation/el/engram/src/server.el).
+      cat <<'EOF'
+foundation/el      | required | substrate: elc + el_runtime + engram source + elp NLG the soul imports
+neuron             | required | the soul (:7770) + engram build; soul.el imports ../foundation/el/elp/src/elp.el
+products/NeuronUI  | required | the app/UI (Kotlin/Compose desktop client; bundles the soul binary)
+products/web       | optional | marketing site + interactive soul-demo
+EOF
+      ;;
+    *) return 1;;
+  esac
+}
+
+is_profile(){ profile_repos "$1" >/dev/null 2>&1; }
+
+ws_dir(){ printf '%s/%s-%s' "$STACK_ROOT" "$1" "$2"; }        # <root>/<profile>-<name>
+ws_branch(){ printf 'sandbox/%s-%s' "$1" "$2"; }             # branch name used in each repo
+manifest(){ printf '%s/.stack-manifest.json' "$1"; }         # <ws>/.stack-manifest.json
+
+# ================================================================ up ===========
+cmd_up(){
+  local profile="$1"; shift
+  local name="" branch="" base_override="" minimal=0
+  [ $# -gt 0 ] && [ "${1#-}" = "$1" ] && { name="$1"; shift; } || die "usage: sandbox $profile <name> [--minimal] [--branch B] [--base REF]"
+  while [ $# -gt 0 ]; do case "$1" in
+    --minimal)       minimal=1; shift;;
+    --branch)        branch="$2"; shift 2;;
+    --base)          base_override="$2"; shift 2;;
+    *) die "unknown flag: $1";;
+  esac; done
+  need git
+  is_profile "$profile" || die "unknown profile: $profile (try: el-stack | neuron-stack)"
+
+  local ws; ws="$(ws_dir "$profile" "$name")"
+  [ -n "$branch" ] || branch="$(ws_branch "$profile" "$name")"
+
+  # -------- pre-flight (fail before creating anything) --------
+  case "$ws" in /tmp/*|/private/tmp/*|/var/tmp/*)
+    die "refusing workspace under a temp dir ($ws) — temp dirs are ablated on compaction; set NSBX_STACK_ROOT to a persistent path";;
+  esac
+  [ -e "$ws" ] && die "workspace already exists: $ws  (sandbox down $profile $name first)"
+
+  # resolve + validate every repo, and pick a base sha per repo, BEFORE touching disk
+  local -a rels roles bases origins wts
+  local line rel role_extra role req origin base wt
+  while IFS= read -r line; do
+    [ -z "${line// }" ] && continue
+    rel="$(printf '%s' "$line"  | cut -d'|' -f1 | xargs)"
+    req="$(printf '%s' "$line"  | cut -d'|' -f2 | xargs)"
+    role="$(printf '%s' "$line" | cut -d'|' -f3- | sed 's/^ *//')"
+    [ "$minimal" -eq 1 ] && [ "$req" = "optional" ] && continue
+    origin="$DEV_ROOT/$rel"
+    git -C "$origin" rev-parse --git-dir >/dev/null 2>&1 || {
+      [ "$req" = "required" ] && die "required repo missing or not a git repo: $origin"
+      warn "skipping optional repo (missing): $rel"; continue; }
+    if [ -n "$base_override" ]; then base="$base_override"; else base="$(git -C "$origin" rev-parse HEAD)"; fi
+    wt="$ws/$rel"
+    [ -e "$wt" ] && die "target worktree path already exists: $wt"
+    rels+=("$rel"); roles+=("$role"); origins+=("$origin"); bases+=("$base"); wts+=("$wt")
+  done < <(profile_repos "$profile")
+  [ "${#rels[@]}" -gt 0 ] || die "no repos resolved for profile $profile"
+
+  log "assembling '$profile' workspace '$name'"
+  info "workspace: $ws"
+  info "branch:    $branch   (created in each repo, off its committed HEAD)"
+  mkdir -p "$ws"
+
+  # -------- create a worktree per repo (mirrored relpath layout) --------
+  local i n="${#rels[@]}"
+  SB_DONE_WTS=(); SB_DONE_ORIGINS=()
+  for ((i=0; i<n; i++)); do
+    rel="${rels[$i]}"; origin="${origins[$i]}"; base="${bases[$i]}"; wt="${wts[$i]}"
+    mkdir -p "$(dirname "$wt")"
+    local gerr
+    if git -C "$origin" show-ref --verify --quiet "refs/heads/$branch"; then
+      gerr="$(git -C "$origin" worktree add "$wt" "$branch" 2>&1)" \
+        || { _rollback; die "git worktree add failed for $rel (existing branch $branch):"$'\n'"    $gerr"; }
+    else
+      gerr="$(git -C "$origin" worktree add -b "$branch" "$wt" "$base" 2>&1)" \
+        || { _rollback; die "git worktree add -b $branch failed for $rel (base $base):"$'\n'"    $gerr"; }
+    fi
+    SB_DONE_WTS+=("$wt"); SB_DONE_ORIGINS+=("$origin")
+    ok "worktree: $rel  ->  ${wt#$ws/}  (branch $branch @ ${base:0:9})"
+  done
+
+  local el_ws="$ws/$EL_REPO_REL"
+  _write_env      "$ws" "$profile" "$name" "$branch" "$el_ws"
+  _write_manifest "$ws" "$profile" "$name" "$branch"
+  _write_build    "$ws" "$profile" "$el_ws"
+  _write_readme   "$ws" "$profile" "$name" "$branch" "$el_ws"
+
+  # -------- summary --------
+  echo >&2
+  printf '%s  STACK WORKSPACE READY — %s / %s%s\n' "$C_BLD" "$profile" "$name" "$C_0" >&2
+  printf '   %-11s %s\n' "workspace" "$ws" >&2
+  printf '   %-11s %s\n' "branch"    "$branch (in each repo)" >&2
+  printf '   %-11s %s\n' "repos"     "$n worktrees, mirrored layout" >&2
+  echo >&2
+  info "get in:      cd $ws && source .stack-env"
+  info "build all:   sandbox build $profile $name        # (or: cd $ws && ./build.sh)"
+  if [ "$profile" = "neuron-stack" ]; then
+    info "isolated engram (opt-in, via nsbx):"
+    info "  nsbx create $profile-$name --source $el_ws && nsbx up $profile-$name"
+  fi
+  info "tear down:   sandbox down $profile $name          # removes all worktrees; branches kept"
+}
+
+# _rollback : remove any worktrees already created this run (globals set by cmd_up)
+SB_DONE_WTS=(); SB_DONE_ORIGINS=()
+_rollback(){
+  local j
+  [ "${#SB_DONE_WTS[@]}" -gt 0 ] && warn "rolling back ${#SB_DONE_WTS[@]} partial worktree(s)"
+  for ((j=${#SB_DONE_WTS[@]}-1; j>=0; j--)); do
+    git -C "${SB_DONE_ORIGINS[$j]}" worktree remove --force "${SB_DONE_WTS[$j]}" 2>/dev/null || rm -rf "${SB_DONE_WTS[$j]}"
+    git -C "${SB_DONE_ORIGINS[$j]}" worktree prune 2>/dev/null || true
+  done
+}
+
+# ---------------------------------------------------------------- writers ------
+_write_env(){
+  local ws="$1" profile="$2" name="$3" branch="$4" el_ws="$5"
+  local elc_dir="$el_ws/lang/dist/platform"
+  cat > "$ws/.stack-env" <<ENV
+# stack env for '$profile/$name' — SOURCE this to work the whole stack together.
+# Pins EL_REPO + PATH at the SANDBOX copy of foundation/el, so elc/elb/runtime and
+# every cross-repo ../foundation/el import resolve INSIDE this workspace.
+# The live mind (:$LIVE_ENGRAM_PORT engram / :$LIVE_SOUL_PORT soul) is deliberately NOT referenced.
+export STACK_NAME="$profile-$name"
+export STACK_PROFILE="$profile"
+export STACK_ROOT_WS="$ws"
+export EL_REPO="$el_ws"
+export PATH="$elc_dir:\$PATH"          # elc, elb (darwin/linux prebuilt) from the sandbox el
+ENV
+  if [ "$profile" = "neuron-stack" ]; then
+    cat >> "$ws/.stack-env" <<ENV
+export NEURON_REPO="$ws/neuron"
+export NEURONUI_REPO="$ws/products/NeuronUI"
+# engram/soul are NOT bound here — bring up an ISOLATED engram via nsbx when needed
+# (nsbx guards the live store & refuses ports :$LIVE_ENGRAM_PORT/:$LIVE_SOUL_PORT):
+#   nsbx create $profile-$name --source \$EL_REPO && nsbx up $profile-$name
+#   nsbx status $profile-$name        # prints the isolated engram URL to point the soul at
+ENV
+  fi
+  # direnv convenience
+  [ -e "$ws/.envrc" ] || printf 'source_env .stack-env 2>/dev/null || source .stack-env\n' > "$ws/.envrc"
+}
+
+_write_manifest(){
+  local ws="$1" profile="$2" name="$3" branch="$4"
+  # emit worktree records from git's own worktree list, filtered to this workspace
+  python3 - "$ws" "$profile" "$name" "$branch" "$DEV_ROOT" <<'PY'
+import json, os, subprocess, sys
+ws, profile, name, branch, dev = sys.argv[1:6]
+repos = []
+for rel in sorted(os.listdir(ws)) if False else []:
+    pass
+# discover worktrees by walking one level of relpaths we created
+def git(root, *a):
+    return subprocess.run(["git","-C",root,*a], capture_output=True, text=True).stdout.strip()
+for dirpath, dirnames, filenames in os.walk(ws):
+    if ".git" in filenames or ".git" in dirnames:
+        rel = os.path.relpath(dirpath, ws)
+        toplevel = git(dirpath, "rev-parse", "--show-toplevel")
+        common   = git(dirpath, "rev-parse", "--git-common-dir")
+        origin   = os.path.realpath(os.path.join(common, ".."))
+        head     = git(dirpath, "rev-parse", "HEAD")
+        repos.append({"rel": rel, "worktree": dirpath, "origin": origin,
+                      "branch": branch, "head": head})
+        dirnames[:] = []  # don't descend into a repo
+repos.sort(key=lambda r: r["rel"])
+json.dump({"profile": profile, "name": name, "branch": branch,
+           "workspace": ws, "repos": repos},
+          open(os.path.join(ws, ".stack-manifest.json"), "w"), indent=2)
+PY
+}
+
+_write_build(){
+  local ws="$1" profile="$2" el_ws="$3"
+  cat > "$ws/build.sh" <<'BUILD'
+#!/usr/bin/env bash
+# build.sh — build the assembled stack together, in dependency order.
+# Generated by `sandbox`. Run from the workspace root (it sources .stack-env).
+set -uo pipefail
+cd "$(dirname "$0")"; source ./.stack-env
+say(){ printf '\033[1m==>\033[0m %s\n' "$*"; }
+ok(){  printf '    \033[32m%s\033[0m\n' "$*"; }
+bad(){ printf '    \033[31m%s\033[0m\n' "$*"; }
+
+# locate an elc that runs on THIS machine (darwin-arm64 / linux-amd64), from the sandbox el
+find_elc(){
+  local d="$EL_REPO/lang/dist/platform"
+  case "$(uname -s)-$(uname -m)" in
+    Darwin-arm64) echo "$d/elc-darwin-arm64";;
+    Linux-x86_64) echo "$d/elc-linux-amd64";;
+    *)            echo "$d/elc";;
+  esac
+}
+ELC="$(find_elc)"; [ -x "$ELC" ] || ELC="$EL_REPO/lang/dist/platform/elc"
+say "elc: $ELC"
+[ -x "$ELC" ] && ok "$("$ELC" 2>&1 | head -1 || echo present)" || { bad "elc not executable"; exit 1; }
+
+# canonical runtime C to link (CI-published release copy; ~8 copies exist in-tree)
+RT="$EL_REPO/lang/releases/v1.0.0-20260501"
+[ -f "$RT/el_runtime.c" ] || RT="$EL_REPO/lang/el-compiler/runtime"
+[ -f "$RT/el_runtime.c" ] && ok "el_runtime: $RT/el_runtime.c" || bad "no el_runtime.c found under $EL_REPO/lang"
+BUILD
+
+  if [ "$profile" = "el-stack" ]; then
+    cat >> "$ws/build.sh" <<'BUILD'
+
+# ---- EL STACK: prove elc + the el stuff (incl. the el-ui framework) build together ----
+say "el-ui framework present: $EL_REPO/ui"
+[ -d "$EL_REPO/ui" ] && ok "framework dir present ($(ls "$EL_REPO/ui" | tr '\n' ' '))" || bad "no ui/ dir"
+
+# end-to-end compiler proof: elc compiles a real, substantial stack source to C,
+# then cc links it against the runtime -> a working native binary.
+B="$(mktemp -d)"
+say "elc end-to-end: compile engram/src/server.el and link a native binary"
+if "$ELC" "$EL_REPO/engram/src/server.el" > "$B/x.c" 2>"$B/elc.err"; then
+  ok "elc -> C ($(wc -c <"$B/x.c" | tr -d ' ') bytes)"
+  if cc -std=c11 -O2 -w -I "$RT" -o "$B/x" "$B/x.c" "$RT/el_runtime.c" -lcurl -lpthread -lm 2>"$B/cc.err"; then
+    ok "cc link ok -> native binary $(ls -lh "$B/x" | awk '{print $5}')  (elc + runtime build together)"
+  else
+    bad "cc link failed:"; grep -i 'error:' "$B/cc.err" | sort -u | head | sed 's/^/      /'
+  fi
+else
+  bad "elc compile failed:"; sed 's/^/      /' "$B/elc.err" | head
+fi
+
+# optional downstream consumer: forge builds on the SDK (make build) — proves the
+# freshly-assembled el SDK still compiles a real downstream repo.
+FORGE="$STACK_ROOT_WS/foundation/forge"
+if [ -f "$FORGE/Makefile" ]; then
+  say "downstream consumer: foundation/forge (make build)"
+  ( cd "$FORGE" && EL_REPO="$EL_REPO" PATH="$EL_REPO/lang/dist/platform:$PATH" make build ) \
+    && ok "forge built against the sandbox SDK" || bad "forge build failed (see above)"
+fi
+say "el-stack build complete"
+BUILD
+  else
+    cat >> "$ws/build.sh" <<'BUILD'
+
+# ---- 1) engram (elc engram/src/server.el -> cc engram.c el_runtime.c), from the sandbox el ----
+say "build engram from $EL_REPO/engram/src/server.el"
+B="$(mktemp -d)"
+if "$ELC" "$EL_REPO/engram/src/server.el" > "$B/engram.c" 2>"$B/elc.err"; then
+  ok "elc -> engram.c ($(wc -c <"$B/engram.c" | tr -d ' ') bytes)"
+  if cc -std=c11 -O2 -w -I "$RT" -o "$B/engram" \
+        "$B/engram.c" "$RT/el_runtime.c" -lcurl -lpthread -lm 2>"$B/cc.err"; then
+    ok "engram binary built: $(ls -lh "$B/engram" | awk '{print $5}')"
+  else
+    bad "engram cc link failed:"; grep -i 'error:' "$B/cc.err" | sort -u | head | sed 's/^/      /'
+  fi
+else
+  bad "engram elc transpile failed:"; sed 's/^/      /' "$B/elc.err"
+fi
+
+# ---- 2) soul (imports ../foundation/el/elp/src/elp.el — resolves to SANDBOX el) ----
+say "soul present + cross-repo import resolves inside the sandbox"
+[ -f "$NEURON_REPO/soul.el" ] && ok "neuron/soul.el present" || bad "no soul.el"
+if [ -f "$EL_REPO/elp/src/elp.el" ]; then
+  ok "../foundation/el/elp/src/elp.el resolves -> $EL_REPO/elp/src/elp.el (sandbox copy)"
+else
+  bad "elp NLG source missing under sandbox el"
+fi
+# soul is a heavy single-TU compile; prove elc parses it rather than a full link
+if "$ELC" "$NEURON_REPO/soul.el" > "$B/soul.c" 2>"$B/soul.err"; then
+  ok "elc compiled soul.el -> $(wc -c <"$B/soul.c" | tr -d ' ') bytes of C (cross-repo imports resolved)"
+else
+  bad "soul.el elc compile failed:"; sed 's/^/      /' "$B/soul.err" | head
+fi
+
+# ---- 3) UI (present + buildable; gradle/JDK21 is heavy so we don't run it here) ----
+say "app/UI present + buildable"
+if [ -f "$NEURONUI_REPO/build.sh" ] || [ -f "$NEURONUI_REPO/gradlew" ]; then
+  ok "NeuronUI build entry present (./build.sh / ./gradlew — needs JDK21; run: cd $NEURONUI_REPO && ./gradlew run)"
+else
+  bad "no NeuronUI build entry"
+fi
+say "neuron-stack build complete (engram compiled, soul compiled, UI present & buildable)"
+BUILD
+  fi
+  chmod +x "$ws/build.sh"
+}
+
+_write_readme(){
+  local ws="$1" profile="$2" name="$3" branch="$4" el_ws="$5"
+  cat > "$ws/README.md" <<MD
+# $profile / $name — combined stack workspace
+
+Assembled by \`sandbox\`. Every constituent repo is a **git worktree** on branch
+\`$branch\`, forked off its origin repo's committed HEAD, laid out at its natural
+relative path so cross-repo \`../foundation/el\` imports resolve **inside this
+workspace** (the sandbox el), never the live tree.
+
+## Get in
+\`\`\`bash
+cd $ws
+source .stack-env      # EL_REPO + PATH now point at the sandbox el
+./build.sh             # build the stack together (or: sandbox build $profile $name)
+\`\`\`
+
+## Layout
+__STACK_LAYOUT__
+
+## Isolation
+- Worktrees only; the live soul/engram (:$LIVE_SOUL_PORT / :$LIVE_ENGRAM_PORT) are never touched.
+- To run against an **isolated engram**, delegate to \`nsbx\` (guards the live store/ports):
+  \`\`\`bash
+  nsbx create $profile-$name --source \$EL_REPO && nsbx up $profile-$name
+  \`\`\`
+
+## Tear down
+\`\`\`bash
+sandbox down $profile $name           # remove every worktree; branch '$branch' kept
+sandbox down $profile $name --delete-branch
+\`\`\`
+MD
+  # fill the layout list from the manifest without embedding backticks in the heredoc
+  python3 - "$ws" <<'PY'
+import json, os, sys
+ws = sys.argv[1]
+d = json.load(open(os.path.join(ws, ".stack-manifest.json")))
+lines = ["- `%s`  <- worktree of %s" % (r["rel"], r["origin"]) for r in d["repos"]]
+p = os.path.join(ws, "README.md")
+txt = open(p).read().replace("__STACK_LAYOUT__", "\n".join(lines))
+open(p, "w").write(txt)
+PY
+}
+
+# ================================================================ down =========
+cmd_down(){
+  local profile="$1" name="$2"; shift 2 || true
+  local del_branch=0
+  while [ $# -gt 0 ]; do case "$1" in
+    --delete-branch) del_branch=1; shift;;
+    *) die "unknown flag: $1";;
+  esac; done
+  need git
+  local ws; ws="$(ws_dir "$profile" "$name")"
+  [ -d "$ws" ] || die "no such workspace: $ws"
+  local mf; mf="$(manifest "$ws")"
+  [ -f "$mf" ] || die "no manifest in $ws (refusing to guess); remove it by hand if intended"
+  local branch; branch="$(python3 -c "import json;print(json.load(open('$mf'))['branch'])")"
+
+  log "tearing down '$profile/$name'  ($ws)"
+  # remove each worktree through its origin repo
+  python3 -c "import json;[print(r['origin']+'\t'+r['worktree']) for r in json.load(open('$mf'))['repos']]" \
+  | while IFS=$'\t' read -r origin wt; do
+      if [ -d "$wt" ]; then
+        git -C "$origin" worktree remove --force "$wt" 2>/dev/null || rm -rf "$wt"
+        git -C "$origin" worktree prune 2>/dev/null || true
+        ok "removed worktree: ${wt#$ws/}"
+      fi
+      if [ "$del_branch" -eq 1 ]; then
+        git -C "$origin" branch -D "$branch" 2>/dev/null && ok "deleted branch $branch in ${origin#$DEV_ROOT/}" || true
+      fi
+    done
+  # drop the (now worktree-free) workspace tree
+  rm -rf "$ws"
+  ok "workspace removed: $ws"
+  [ "$del_branch" -eq 1 ] || info "branch '$branch' kept in each repo (use --delete-branch to drop)"
+  ok "down '$profile/$name' complete (live untouched)"
+}
+
+# ================================================================ build ========
+cmd_build(){
+  local profile="$1" name="$2"; local ws; ws="$(ws_dir "$profile" "$name")"
+  [ -x "$ws/build.sh" ] || die "no build.sh in $ws (is it assembled? sandbox $profile $name)"
+  exec "$ws/build.sh"
+}
+
+# ================================================================ list/status ==
+cmd_list(){
+  [ -d "$STACK_ROOT" ] || { info "no stack workspaces (root $STACK_ROOT absent)"; return 0; }
+  local mf found=0
+  for mf in "$STACK_ROOT"/*/.stack-manifest.json; do
+    [ -f "$mf" ] || continue; found=1
+    python3 -c "import json;d=json.load(open('$mf'));print('  %-22s %-8s %2d repos  branch=%s'%(d['profile']+'/'+d['name'],'',len(d['repos']),d['branch']))" 2>/dev/null
+  done
+  [ "$found" -eq 1 ] || info "no assembled stack workspaces under $STACK_ROOT"
+}
+
+cmd_status(){
+  local profile="$1" name="$2"; local ws; ws="$(ws_dir "$profile" "$name")"
+  local mf; mf="$(manifest "$ws")"; [ -f "$mf" ] || die "no such workspace: $ws"
+  log "stack '$profile/$name'"; info "workspace: $ws"
+  python3 - "$mf" <<'PY'
+import json,sys,subprocess
+d=json.load(open(sys.argv[1]))
+print(f"    branch:    {d['branch']}")
+for r in d['repos']:
+    st=subprocess.run(["git","-C",r["worktree"],"status","--porcelain"],capture_output=True,text=True).stdout
+    n=len([l for l in st.splitlines() if l.strip()])
+    print(f"    {r['rel']:<20} {r['head'][:9]}  {'clean' if n==0 else str(n)+' changed'}")
+PY
+}
+
+# ================================================================ usage/main ===
+usage(){ cat >&2 <<EOF
+${C_BLD}sandbox${C_0} — assemble a WHOLE Neuron stack into one combined worktree workspace,
+wired to build together on an isolated clean base. Multi-repo sibling of ${C_BLD}nsbx${C_0}.
+
+  ${C_CYN}sandbox el-stack     <name>${C_0} [--minimal]   elc + EL language + el-ui framework + tooling (+ SDK consumers)
+  ${C_CYN}sandbox neuron-stack <name>${C_0} [--minimal]   runtime/soul + engram + app/UI  (the full product)
+  ${C_CYN}sandbox build  <profile> <name>${C_0}           build the assembled stack together (runs its build.sh)
+  ${C_CYN}sandbox status <profile> <name>${C_0}           inspect one workspace
+  ${C_CYN}sandbox list${C_0}                               list assembled workspaces
+  ${C_CYN}sandbox down   <profile> <name>${C_0} [--delete-branch]   tear down (remove worktrees; branch kept)
+
+  Flags:  --minimal  only the required repos    --branch B  branch name    --base REF  fork point
+  Env:    NSBX_STACK_ROOT (workspace root, default \$DEV_ROOT/stack-worktrees)   NEURON_DEV_ROOT
+
+Each constituent repo becomes a git worktree at its natural relpath inside the
+workspace, so cross-repo ../foundation/el imports resolve to the SANDBOX el. The
+live soul/engram (:$LIVE_SOUL_PORT / :$LIVE_ENGRAM_PORT) are never touched; isolated-engram
+bring-up is delegated to nsbx.
+EOF
+}
+
+main(){
+  local cmd="${1:-}"; shift || true
+  case "$cmd" in
+    el-stack|neuron-stack) cmd_up "$cmd" "$@";;
+    up)        [ $# -ge 1 ] || die "usage: sandbox up <profile> <name>"; local p="$1"; shift; cmd_up "$p" "$@";;
+    down)      [ $# -ge 2 ] || die "usage: sandbox down <profile> <name>"; cmd_down "$@";;
+    build)     [ $# -ge 2 ] || die "usage: sandbox build <profile> <name>"; cmd_build "$@";;
+    status)    [ $# -ge 2 ] || die "usage: sandbox status <profile> <name>"; cmd_status "$@";;
+    list|ls)   cmd_list "$@";;
+    ""|-h|--help|help) usage;;
+    *) die "unknown command: $cmd (try: sandbox help)";;
+  esac
+}
+main "$@"