feat(sync): device-to-device inventory sync, end to end

Wires the encrypted sync transport into a working replication between one
identity's own devices, reusing the CRDT-ready model (HLC + tombstones +
LWW) the data model was built for.

- SocialSession gains a 5th interface, sync, over the shared connection,
  built with a per-install deviceId + the app's inventory namespace.
- DeviceIdStore: a stable random per-install id in the keystore — distinct
  from the seed-derived CRDT node id, which is IDENTICAL across a user's
  devices (so it couldn't tell them apart / they'd clobber each other's
  replaceable record).
- InventorySnapshotIO (implemented by ExportImportService): build/apply the
  inventory as the SAME interchange JSON as backups but UNSEALED — the sync
  transport does the encryption, so it must not double-seal. Apply = the
  existing idempotent LWW importInventory.
- SyncService: on (re)connect publishes this device's snapshot + subscribes
  to the others'; a local edit (debounced, off tableUpdates) republishes;
  incoming snapshots merge idempotently; skips its own echo; a byte check
  skips re-publishing an unchanged snapshot so the ping-pong converges.
  Foreground only, like the inbox.
- DI/Bootstrap/switchSocialAccount wire and lifecycle it per identity.

Tests: DeviceIdStore, ExportImportService snapshot round-trip (unsealed +
idempotent), SyncService.handleRemote (applies others', skips own).

Follow-ups: deltas vs full-snapshot pushes; background sync; conflict UI.

apps/app_seeds/lib/services/social_service.dart

@@ -9,14 +9,23 @@ import 'package:commons_core/commons_core.dart';
 /// social sessions on demand. Local-first: constructing this is cheap and
 /// offline; nothing connects to a relay until [openSession] is called, so the
 /// app runs fully without network and the social layer only enriches.
+/// The app-data namespace this identity's devices sync their inventory under.
+/// Lives here (app layer), not in `commons_core`, which stays seed-agnostic.
+const kInventorySyncNamespace = 'org.comunes.tane/inventory';
+
 class SocialService {
   SocialService({
     required this.identity,
     this.account = 0,
     this.rootSeedHex = '',
+    this.deviceId = '',
     List<String> relays = const [],
   }) : relays = List.unmodifiable(relays);
 
+  /// This install's device id (for device-to-device sync). Empty in tests /
+  /// when sync isn't wired.
+  final String deviceId;
+
   /// The derived Nostr identity (secp256k1). Same key across reinstalls that
   /// restore the same seed (for a given [account]).
   final NostrIdentity identity;
@@ -43,6 +52,7 @@ class SocialService {
   static Future<SocialService> fromRootSeedHex(
     String rootSeedHex, {
     int account = 0,
+    String deviceId = '',
     List<String> relays = const [],
   }) async {
     final identity = await NostrKeyDerivation.deriveFromSeed(
@@ -53,6 +63,7 @@ class SocialService {
       identity: identity,
       account: account,
       rootSeedHex: rootSeedHex,
+      deviceId: deviceId,
       relays: relays,
     );
   }
@@ -74,18 +85,23 @@ class SocialService {
   /// skipped; throws only when none are reachable. Caller closes it.
   Future<SocialSession> openSession(List<String> relayUrls) async {
     final pool = await RelayPool.connect(relayUrls, identity: identity);
-    return SocialSession(pool);
+    return SocialSession(pool, deviceId: deviceId);
   }
 }
 
 /// One relay channel with the three transports on top — the
 /// "one channel, three interfaces" shape, at the app layer.
 class SocialSession {
-  SocialSession(this._channel)
+  SocialSession(this._channel, {String deviceId = ''})
       : offers = NostrOfferTransport(_channel),
         messages = NostrMessageTransport(_channel),
         trust = NostrTrustTransport(_channel),
-        profile = NostrProfileTransport(_channel);
+        profile = NostrProfileTransport(_channel),
+        sync = NostrSyncTransport(
+          _channel,
+          namespace: kInventorySyncNamespace,
+          deviceId: deviceId,
+        );
 
   final NostrChannel _channel;
 
@@ -94,6 +110,9 @@ class SocialSession {
   final TrustTransport trust;
   final ProfileTransport profile;
 
+  /// Device-to-device inventory sync for THIS identity's own devices.
+  final SyncTransport sync;
+
   Future<void> close() => _channel.close();
 }