chore(engagement): regenerate the manifest for coreApi ^1.21.0

#72 raised coreApi to ^1.21.0 without regenerating
engagement-triggers.json, which embeds it, so check:engagement has
failed since. That went unseen on #72 (its server-tests job died on a
full runner disk before the check) and surfaced on the cutover (#73).
Only the coreApi line changes; no trigger, stream, audience or rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

engagement-triggers.json

@@ -1,6 +1,6 @@
 {
   "_comment": "Generated freeze of module-rust's engagement contract (docs/modules/rust/PLAN.md §25). Regenerate with `npm run engagement:manifest` in server/. A renamed variable, a changed type or a widened ceiling breaks stored templates and rules, so the diff here is the review signal.",
-  "coreApi": "^1.20.0",
+  "coreApi": "^1.21.0",
   "triggers": [
     {
       "id": "rust.base.destroyed",