docs: sync governance and validation with hardening pass

Update contributor, security, validation, and compute handoff documentation to reflect new runtime safeguards, CI gates, and expected regression checks.

Made-with: Cursor

Agents/COMPUTE_PLAN.md

@@ -2,7 +2,7 @@
 
 **Owner:** Agent team  
 **Status:** Active  
-**Last updated:** 2026-04-16  
+**Last updated:** 2026-04-16 (audit hardening pass)  
 
 **Canonical math:** [MATH_SPEC.md](MATH_SPEC.md)  
 **Agent rules:** [AGENTS.md](../AGENTS.md) at repo root  
@@ -161,6 +161,10 @@ This section is the execution plan for the next pass, optimized for "feature-ric
 ### 8.1 Priority 1 — Correctness gates first
 
 - Build a field-sensitivity harness (per mapped input field, ±1% perturbation).
+- Implemented in this pass:
+  - safer stdin/file JSON ingestion guards in `solver-c/src/json_stdin.c`, `solver-c/src/main.c`, `solver-c/src/main_fea.c`,
+  - API 4xx envelope normalization with machine-readable `code`,
+  - GUI stale-run guard for async solve execution.
 - Enforce invariants in solver outputs:
   - finite values everywhere,
   - gas fraction bounds,