feat(events): route all backend events through a single event bus and store

Adds src/services/bus (the only listen/emit site: one handler per contract
event plus the frontend-internal test-all event) and the React
context/reducer app store. Migrates the five scattered subscriptions
(layout events, pending failures, proxy refresh, profile update/timer,
test-all), deletes the old glue (events.ts, use-listen, use-layout-events,
LoadingCacheProvider), moves run state into the store, and adds an
architecture gate script wired into CI. Behavior is unchanged; the
run-state query's poll/focus safety net is preserved by the store
provider.

.github/workflows/frontend-check.yml

@@ -86,6 +86,10 @@ jobs:
         if: steps.check_frontend.outputs.frontend == 'true'
         run: pnpm knip:check
 
+      - name: Architecture gates (listen/emit only in the event bus)
+        if: steps.check_frontend.outputs.frontend == 'true'
+        run: node scripts/check-architecture.mjs listen emit
+
   contract-parity:
     runs-on: ubuntu-22.04
     steps: