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/src/identity/actor.resolver.ts

@@ -68,10 +68,10 @@ export class ActorResolver {
     );
   }
 
-  async ensureParticipant(threadId: string, actorId: string): Promise<void> {
+  async ensureParticipant(threadId: string, actorId: string, participantRole = 'MEMBER'): Promise<void> {
     await this.prisma.iiosThreadParticipant.upsert({
       where: { threadId_actorId: { threadId, actorId } },
-      create: { threadId, actorId },
+      create: { threadId, actorId, participantRole },
       update: {},
     });
   }