feat(FN-3707): implement mesh auth snapshot replication across core, engine

The merge delivers two major features: mesh auth snapshot synchronization across nodes (FN-3707 steps 2/4/5, including core types, central-core plumbing, peer-exchange updates, and settings/node sync route helpers) and a bundled roadmap plugin with chat UX improvements (FN-3162, FN-3756, FN-3771, FN

Fusion-Task-Id: FN-3707

packages/core/src/central-core.ts

@@ -3171,9 +3171,14 @@ export class CentralCore extends EventEmitter<CentralCoreEvents> {
     return createAuthMaterialSnapshot(providerAuth);
   }
 
-  applyAuthMaterialSnapshot(snapshot: AuthMaterialSnapshot): Record<string, ProviderAuthEntry> {
+  applyAuthMaterialSnapshot(snapshot: AuthMaterialSnapshot): { success: true; authCount: number; providerAuth: Record<string, ProviderAuthEntry> } {
     validateSnapshotEnvelope(snapshot);
-    return { ...(snapshot.payload.providerAuth ?? {}) };
+    const providerAuth = { ...(snapshot.payload.providerAuth ?? {}) };
+    return {
+      success: true,
+      authCount: Object.keys(providerAuth).length,
+      providerAuth,
+    };
   }
 
   // ── Settings Sync API ─────────────────────────────────────────────────