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

README.md

@@ -93,6 +93,12 @@ cd solver-api && npm install && npm run dev
 cd gui-ts && npm install && npm run dev
 ```
 
+Optional API hardening for browser clients:
+
+```bash
+cd solver-api && CORS_ORIGINS="http://localhost:5173" npm run dev
+```
+
 - API: `http://localhost:4400/health`  
 - GUI: `http://localhost:5173`