chore: add check-plugin-api.mjs contract check + verify script

- Copy check-plugin-api.mjs into scripts/ (from WorkBuddy diagnosis)
- Add 'verify' script: node scripts/check-plugin-api.mjs
- 5-section contract check: dependency versions vs DSH runtime,
  import symbols vs runtime exports, slot registration, removed/deprecated
  APIs, inline package dependency closure
- All 5 sections pass (4 deprecation warnings, non-blocking)

package.json

@@ -86,6 +86,7 @@
     "test:mount:aggregate": "bash scripts/e2e-aggregate-mount.sh",
     "clean": "node -e \"require('node:fs').rmSync('lib',{recursive:true,force:true})\"",
     "check:consumer-types": "bash scripts/check-consumer-types.sh",
+    "verify": "node scripts/check-plugin-api.mjs",
     "lint": "eslint .",
     "switch": "bash scripts/switch.sh",
     "switch:status": "bash scripts/switch.sh --status",