feat(iios): policy-enforced membership + threaded replies + dev IdP (generic)

Enriches the platform PLANE, not the kernel:
- DevOpaPort: the dev OPA stub now evaluates a small policy table (behind the
  same opa.decide) — DM capped at 2, add-participant requires member/group-admin,
  governed self-join for membership threads. Real OPA swaps in unchanged.
- Dev IdP login (POST /v1/dev/login) issues the same JWT claims a real IdP would.
- PolicyDeniedFilter maps fail-closed denials to HTTP 403.

Generic kernel additions (no chat vocabulary — 'dm'/'group' live only as OPA
policy + an opaque thread attribute):
- MessageService.addParticipant (governed membership by userId), governed
  openThread self-join (scoped to threads with a membership attribute),
  parentInteractionId on send (reply link), and a generic listThreads.
- REST: GET /v1/threads, POST /v1/threads, POST /v1/threads/:id/participants;
  socket add_participant + membership/parentInteractionId. ensureParticipant
  gains a role.

Tests: dev-opa.port.spec + message.spec (DM cap / group admin / governed join /
listThreads / reply). smoke-membership.mjs; realtime smokes updated for governed
join. 175 unit tests + all smokes green; kernel free of dm/group literals.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/scripts/smoke-realtime.mjs

@@ -44,10 +44,11 @@ const alice = connect('alice');
 const bob = connect('bob');
 
 try {
-  // Alice creates a thread; Bob joins it.
+  // Alice creates a thread; membership is governed, so Alice ADDS Bob (he can't self-join).
   const opened = await alice.emitWithAck('open_thread', {});
   const threadId = opened.threadId;
   assert(!!threadId, `Alice created thread ${threadId}`);
+  await alice.emitWithAck('add_participant', { threadId, userId: 'bob' });
   await bob.emitWithAck('open_thread', { threadId });
 
   // Alice sends; Bob should receive it live.