runtime: the link set is multi-file — name it once, ship all of it

el_runtime.c was created 2026-05-03 as an explicitly temporary build shim. It
was deleted that afternoon ("runtime is 100% native El") and restored 25 minutes
later "UNTIL the compiler is updated to emit #include el_seed.h". The `until`
never came. 3.5 months on it is 20,527 lines, and nothing was ever set up to
notice — a file scheduled for deletion gets no owner, no budget, no boundary.

What kept it growing is not inertia, it is an instruction. lang/AGENTS.md said
el_runtime.c "is the authoritative single-file link target ... THIS IS WHERE A
NEW C BUILTIN'S IMPLEMENTATION MUST CURRENTLY LIVE TO BE LINKABLE", and made it
step 1 of the add-a-builtin recipe. That is false. Placement is a link-time
concern: builtin_arity maps NAME -> ARITY INT only, the El name is emitted as
the exact C symbol, and `ld` resolves it — the compiler cannot tell which .c a
symbol came from. `nm lang/dist/platform/elc` on the shipped compiler already
shows T _engram_geo_reify_index_new, T _vindex_insert, T _engram_think,
T _engram_reason_abduce: it is linked from ten translation units today. In a
repo where agents write most of the code, a false instruction in the instruction
file is the forcing function. The file grew because the recipe said to grow it.

The multi-file runtime is therefore already real, and the docs and the
distribution never caught up — which left a live, shipped bug:

  * Linking el_runtime.c alone FAILS at `ld` (undefined engram_ground_json,
    engram_activate_inner, eg_find_relation, cog_assert_two_axis, ...) because
    el_runtime.c #includes six engram headers and calls into all six siblings.
  * sdk-release.yaml shipped el_runtime.c/.h + engram_store.c/.h and none of the
    other five required .c files, so downstream consumers of the el-runtime-c
    Artifact Registry package and of install.sh got a lib/ that cannot link.
  * .githooks/pre-commit linked el_runtime.c alone with stderr to /dev/null, so
    it reported all 13 native suites as FAILED with the real ld error invisible.
  * AGENTS.md's self-host recipe compiled el-compiler/runtime/el_runtime.c — a
    path the same file's "DO NOT EDIT" list names as a lagging fork.

The root fix is to stop writing the list down eight times:

  * lang/runtime/SOURCES — the canonical link set, in one place, in link order.
  * scripts/el-runtime-sources.sh — prints it, optionally prefixed; --check
    fails loudly on a missing file, --headers for the shipped headers.
  * Every link line in AGENTS.md, lang/AGENTS.md, DESIGN.md, lang/spec/language.md,
    the three workflows and the pre-commit hook now reads that one list.
  * Adding a concern's .c is one line in SOURCES, so a new builtin no longer has
    to be appended to el_runtime.c just because appending was the cheaper edit.

Distribution: ship the siblings rather than amalgamate. Amalgamation needs a new
tool and contradicts DESIGN.md's compile-once-link-many; the siblings are already
independently authored and independently tested (engram/test/*.sh link subsets
directly), and engram_store.c was already shipped, so this completes a mechanism
that existed rather than inventing one. Source is also a superset: a consumer
that wants one file can concatenate, one that wants separate TUs cannot undo an
amalgamation. el-runtime-c/-h stay for backward compatibility; el-runtime-src is
added carrying the complete set plus SOURCES.

lang/AGENTS.md now points new C builtins at the concern-owning .c and states
plainly that the compiler cannot tell which .c a symbol came from, with the nm
evidence. AGENTS.md's "reconcile which is canonical (verify)" note is resolved:
neither file supersedes the other, the canonical unit is the set.

Verified locally (the bar; not CI):
  * engram/src/server.el compiles and links against the SOURCES set.
  * Compile-once-link-many into libel.a links the same program.
  * elb builds from the corrected recipe.
  * Self-host fixpoint byte-identical (11,110 lines, stage2 == stage3) built
    with the SOURCES-driven link line.
  * pre-commit hook: 0 of 13 native suites passing -> 8 of 13.

The 5 still-failing suites are PRE-EXISTING and untouched here: test_fs
(fs_list_json undeclared), test_state (state_has, state_get_or undeclared),
test_json (json_build_array/json_build_object/json_escape_string undefined),
test_time (now_ns undefined), test_env (1 assertion). Builtins registered in
builtin_arity with no implementation or no declaration anywhere — the same
recipe defect, now visible because the linker error is no longer suppressed.

Not attempted: making elc emit #include el_seed.h and dropping elb's hardcoded
runtime path. That is the correct long-term fix and finishes the 2026-05-03
migration, but it touches codegen and self-hosting and belongs in its own change.

lang/AGENTS.md

@@ -77,14 +77,28 @@ This is where almost all work belongs. El programs are source files that get com
 
 This is the self-contained C OS-boundary layer. It provides the `__`-prefixed primitives that compiled El programs call: libcurl HTTP, pthreads, filesystem I/O, arena allocation, etc. It is **not generated** — it is maintained by hand.
 
-The runtime is native El (`runtime/*.el`) over a C OS-boundary. **Status (verified 2026-08-15):** the migration to a seed-only boundary is *in progress, not done*. Two files exist:
-- `runtime/el_runtime.c` (~860 KB) — **LIVE**. Holds the engram store (`EngramStore engram_global`) plus the `http_*`/`json_*`/`state_*`/`engram_*` impls. It is the authoritative single-file link target for the compiler, and `tools/install.sh` compiles it into `libel.a`. This is where a new C builtin's *implementation* must currently live to be linkable.
-- `runtime/el_seed.c` — the intended hand-maintained `__`-prefixed seed (thin wrappers over the above). It is compiled alongside `el_runtime.c` by `tools/install.sh`, but does **not** compile standalone yet (see the build-path caveat under "Rebuilding the Compiler").
+The runtime is native El (`runtime/*.el`) over a C OS-boundary. **Status (verified 2026-08-16):** the migration to a seed-only boundary is *in progress, not done*.
+
+**The runtime is MULTI-FILE. There is no single-file link target and there has not been one for months.** The canonical link set is listed once, in **`runtime/SOURCES`**, and printed by `scripts/el-runtime-sources.sh`. It currently holds ten translation units: `el_runtime.c`, `el_seed.c`, the six `engram_*.c` concern files, and `eg_cosine_batch{,_strategy_cpu}.c`.
+
+- `runtime/el_runtime.c` (~940 KB, 20.5k lines) — **LIVE, and oversized.** It began life on 2026-05-03 as a temporary build shim: it was deleted that afternoon ("runtime is 100% native El") and restored 25 minutes later, explicitly "UNTIL the compiler is updated to emit `#include el_seed.h`". That `until` never arrived, and in the 3.5 months since, the file doubled. **It is not a volatility unit — it is a dumping ground.** ~47.5% of it is engram code that belongs in the six sibling files that already exist. Do not add to it. See "Where a new C builtin goes" below.
+- `runtime/el_seed.c` — the intended hand-maintained `__`-prefixed seed (thin wrappers over the above).
+- `runtime/engram_{store,vindex,geometry,reason,verify,cognition}.c` — the engram concerns, each with its own header. `el_runtime.c` `#include`s all six headers and makes hard cross-TU calls into all six.
+
+> **Linking `el_runtime.c` alone does not work and has not for months.** It fails at `ld` with undefined symbols (`engram_ground_json`, `engram_activate_inner`, `eg_find_relation`, `cog_assert_two_axis`, …). Any recipe, script, or CI step that names `el_runtime.c` by itself is stale — replace it with `$(scripts/el-runtime-sources.sh lang/runtime)`.
 
 **Only edit these when you genuinely need OS-level access** (raw sockets, GPU calls, new libcurl features, a new engram store op). For everything else, write El.
 
+#### Where a new C builtin goes
+
+**Put it in the `.c` that owns the concern — NOT in `el_runtime.c`.**
+
+*Placement is a link-time concern. The compiler cannot tell which `.c` a symbol came from, and never could.* `builtin_arity` in `el-compiler/src/codegen.el` maps NAME → ARITY INT and nothing else (~413 entries); the El name is emitted as the exact C symbol and resolved by `ld`. Proof, if you want it: `nm lang/dist/platform/elc` on the *shipped* compiler shows `T _engram_geo_reify_index_new` (defined in `engram_geometry.c`), `T _vindex_insert` (`engram_vindex.c`), `T _engram_think` (`engram_cognition.c`), `T _engram_reason_abduce` (`engram_reason.c`). **The shipped compiler is already linked from ten translation units.** A builtin defined in a sibling `.c` is exactly as linkable as one defined in `el_runtime.c`.
+
+Choose the file by concern: engram store ops → `engram_store.c`; index → `engram_vindex.c`; geometry/priming → `engram_geometry.c`; reasoning → `engram_reason.c`; grounding/consistency → `engram_verify.c`; think/stance → `engram_cognition.c`. **If no existing file owns it, create one** — add the `.c` to `runtime/SOURCES` (one line) and every build path picks it up. For a builtin that belongs to a downstream program rather than the runtime, declare `c_source "path/to/file.c"` in that program's `manifest.el`; `elb` already links it (`parse_manifest_c_sources`, `lang/elb.el:82`).
+
 When you add a C builtin (verbatim-emit recipe — the El name is emitted as the exact C symbol; `builtin_arity` is an arity guard only, not a dispatch table):
-1. Implement the C function in `el_runtime.c` (and declare it in `el_runtime.h`).
+1. Implement the C function in the **concern-owning `.c`** (and declare it in that file's `.h`). Add the file to `runtime/SOURCES` if it is new. Only put it in `el_runtime.c` if it is genuinely EL core (val/str/map/list/arena) — that is ~8% of what is in there today.
 2. Add a `__`-prefixed thin wrapper in `el_seed.c` and declare it in `el_seed.h`.
 3. Add the name to `builtin_arity` in `el-compiler/src/codegen.el` — add **both** the plain and `__`-prefixed spellings.
 4. Rebuild the elc binary (see below) and confirm the self-host fixpoint is byte-identical.
@@ -125,7 +139,9 @@ diff elc-new.c elc-verify.c   # must be identical
 mv dist/platform/elc-new dist/platform/elc
 ```
 
-> **Build-path caveat (verified 2026-08-15).** `el_seed.c` is the intended hand-maintained OS-boundary seed, but it does **not** compile standalone under modern clang: it wraps ~16 unprefixed `el_runtime.c` symbols (`http_serve`, `json_*`, `state_*`, `http_response`) without prototypes, and clang treats implicit declarations as errors (C99+). The productionised install (`tools/install.sh`) builds `libel.a` from **both** `el_seed.o` + `el_runtime.o` together, which is why linking succeeds there. To make `el_seed.c` build on its own, add prototypes for those symbols (or `#include "el_runtime.h"`, reconciling the `__http_serve` return-type mismatch first). Until then, `el_runtime.c` is the authoritative single-file link target for the compiler.
+> **Build-path caveat (verified 2026-08-15).** `el_seed.c` is the intended hand-maintained OS-boundary seed, but it does **not** compile standalone under modern clang: it wraps ~16 unprefixed `el_runtime.c` symbols (`http_serve`, `json_*`, `state_*`, `http_response`) without prototypes, and clang treats implicit declarations as errors (C99+). The productionised install (`tools/install.sh`) builds `libel.a` from **both** `el_seed.o` + `el_runtime.o` together, which is why linking succeeds there. To make `el_seed.c` build on its own, add prototypes for those symbols (or `#include "el_runtime.h"`, reconciling the `__http_serve` return-type mismatch first).
+>
+> **There is no single-file link target.** *(Corrected 2026-08-16 — this paragraph previously ended "`el_runtime.c` is the authoritative single-file link target for the compiler". Measured: that is false. Linking `elc-new.c` against `runtime/el_runtime.c` alone fails at `ld` with undefined `engram_ground_json`, `engram_activate_inner`, `eg_find_relation`, `cog_assert_two_axis`, and others, because `el_runtime.c` `#include`s six engram headers and calls into all six sibling `.c` files.)* Link the set in `runtime/SOURCES` via `$(../scripts/el-runtime-sources.sh runtime)`.
 
 After changing `el_seed.c` only (no El source changes), rebuild downstream programs but do NOT need to rebuild the compiler binary itself — the seed is linked at the application level, not the compiler level.