refactor(#74) P6: bump to v0.40.0 + changelog

- package.json: 0.39.0 → 0.40.0.
- CHANGELOG.md: new v0.40.0 entry documenting the modular LLM-ability
  system (the behavior-preserving extraction of the 8 per-feature
  abilities into the ordered ABILITIES registry), with the guarantees
  (byte-identical prompts/mutations/stub, zero client changes, no new
  deps, shared/ stays dep-free) and the new registry/golden tests.

Verified: typecheck + lint + 522 unit tests + build all green.

Fixes #74

package.json

@@ -1,6 +1,6 @@
 {
   "name": "history",
-  "version": "0.39.0",
+  "version": "0.40.0",
   "private": true,
   "type": "module",
   "description": "AI-driven alternate-history simulator — the LLM is the world engine (Pax Historia style).",