chore(engine): make dist/soul.c drift a build failure instead of a silent ship (#133)

#133 regenerated the amalgam once and said so itself: 'Nothing in the tree
regenerates this file. Only a human running the recipe. It lags in batches, never
per-change, and it will drift again.'

It drifted again. Every binary deployed on 2026-08-09 was built by build-soul.sh
from a scratch amalgam that never touches dist/soul.c, so the committed build
input fell 2,761 bytes behind the sources by a different route than #133 describes.

Auto-regeneration is not available: the CI workflow records that elc needs 24GB+
of virtual memory and would OOM the runner. So the build cannot regenerate the
file. It can refuse to compile a stale one, for free and with no compiler.

tools/soulc-stamp.sh records a content fingerprint of every .el source at the
moment the amalgam is generated. --check recomputes and compares; divergence exits
1 and names the changed files and the recipe. Wired into CI ahead of the compile.

dist/soul.c regenerated from current sources: 1,176,361 -> 1,179,122 bytes, 1,247
inlined bodies (gate wants >=1200), and verified to compile clean at 903,552 bytes.

Demonstrated to FAIL on the bad input, per postmortem 0004's rule that a gate which
only passes on good input proves nothing:
  fresh stamp        -> OK,   exit 0
  one .el modified   -> FAIL, exit 1, names memory.el
  source restored    -> OK,   exit 0

Refs #133, #111

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -63,6 +63,16 @@ jobs:
           cp vendor/el-runtime/v1.0.0-20260501/el_runtime.h /opt/el/runtime/el_runtime.h
           echo "El runtime PINNED to v1.0.0-20260501: $(ls /opt/el/runtime/)"
 
+      # neuron#133: CI compiles dist/soul.c, NOT the .el sources. On 2026-08-07 a
+      # build off main would have shipped an engine with none of five merged fixes,
+      # including a P0 safety fix, while main's source read as correct. The runner
+      # cannot regenerate the amalgam (elc needs 24GB+ virtual memory), but it can
+      # refuse to compile a stale one. Fails loudly with the recipe in the message.
+      - name: Verify dist/soul.c matches the sources
+        run: |
+          chmod +x tools/soulc-stamp.sh
+          ./tools/soulc-stamp.sh --check
+
       - name: Build neuron soul binary
         run: |
           RUNTIME=/opt/el/runtime