engram: fix lazy-embed index gap (#20) and make activate's cosine scan lazy; extract vindex harvest primitive with a bench/oracle harness

Adds an O(1) "seen" bitmap so lazily-embedded older nodes get picked up
incrementally instead of only on a full rebuild (embed-gap #20).

Replaces engram_activate's O(N*D) cosine prescan with a lazy-memoized
cosine cache (eg_cosq_at), proven bit-identical to the old path.

Extracts a clean vindex_harvest_from_store primitive (read-only vector
harvest, careful malloc/ownership/error-path handling) reused by both
index-build and the new vindex_bench.c — a read-only proof harness
comparing brute-force vs HNSW recall/latency on both the real store and
synthetic data.

.nsbx-env intentionally excluded — local sandbox config (ports, paths,
dev-only placeholder key), not checked in.

lang/AGENTS.md

@@ -4,6 +4,37 @@ El is a self-hosting, statically-typed language that compiles to C. This file or
 
 ---
 
+## Current work in this worktree — the API reshape / decorated seam (IN PROGRESS, 2026-08-14)
+
+This is the `api-reshape` worktree. The build here reshapes Neuron's external
+surface and how it is *declared* — proven on isolated dev-port clones only; **live
+prod engram `:8742` is untouched and nothing is promoted.** Full framing lives in
+`neuron/docs/architecture/06-cognitive-architecture.md` (Update — 2026-08-14 deep
+night) and `02-components.md §5`.
+
+- **Surface collapse.** The ~90 noun-organized CRUD MCP tools collapse to a few
+  **geometry ops** — `read` (the *vantage-read*: re-origin + salience/recency +
+  an **aperture** → a bounded slice, curing the whole-self dump), `write`,
+  `relate`, `supersede` (evolve/tombstone/promote, never a hard delete) — plus the
+  agentic primitives `think`/`attend`/`learn`/`ground`/`assert`. The old noun is a
+  `type` parameter. Implemented in `tools/api-reshape/surface.el` with a parity
+  harness (`parity.sh`); aperture proven to bound output. **Not yet:** compiled
+  into the MCP server, hot-swap, all-alias dispatch.
+- **Decorated seam.** `@route(path,method,…)` makes codegen synthesize
+  `el_route_dispatch` (replacing the hand-written `handle_request` if-else) —
+  proven decorate→serve on `:8951`. `@manager`/`@engine`/`@accessor` are **parsed
+  but structurally inert** in the shipped compiler today; the `@route` codegen
+  lives on the **unmerged branch `feat/el-route-decorators`**. Telemetry-emit and
+  dharma-bus auto-wiring at the boundary are **staged, not shipped**. In-process,
+  an `@accessor` reaches the engram via **`engram_*` builtins**, not `http_get`.
+
+**Do not edit** the protected build sources while this is in flight:
+`el-compiler/src/codegen.el`, `el-compiler/runtime/el_seed.c` (and the archived
+`legacy/el_runtime.c`), the `runtime/engram_*.c` boot files, and `surface.el`
+(when present in the reshape tree) — these are owned by the build agents.
+
+---
+
 ## What El Is
 
 El compiles `.el` source → C → native binary. Every El value is `el_val_t` (int64_t). Strings are heap pointers cast through int64_t. The compiler is written in El (self-hosting).