Restructure step 1: lasting identity, presence and protocol 16

Server and core: version ranges on both sides with a coded refusal; a
client id maps to the same member and token for the server's life; a
drop is `gone` for a 10-minute grace keeping ready, presence, queue
items, relay and avatar; `bye` leaves at once; ordered takeover (seq,
replaced/stale); presence joining/in/away/gone with one PresencePolicy;
joiners never pause playback and hold a held gate at most ~15 s;
state.hold; feed ids, seq and replay; the relay lives through its
source's grace and continues on re-dial; queue `by` lasting, auto
items and cancel; MediaRef carried; client ids never logged.

App: persisted client id and seq; Connection with unstable/lost/
replaced states, backoff with jitter, no lost/back pair per flap; a drop
keeps the session (state, player, media, relay source); presence in the
strip and members card; sit out = away intent; feed matched by id;
auto sit-out (idle when not playing, minimised for a while; never when
alone) replaces auto-unready; bye on close and Leave, not on update
restart; relay source re-dials through its grace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/test/server_isolate_test.dart

@@ -38,12 +38,7 @@ class _Raw {
 
   static Future<_Raw> join(int port, String name, {String secret = ''}) async {
     final r = _Raw(await WebSocket.connect('ws://127.0.0.1:$port/sync'));
-    r.ws.add(jsonEncode(HelloMessage(
-            proto: RelayConstants.protoVersion,
-            secret: secret,
-            user: HelloUser(name: name),
-            media: const HelloMedia(name: '', size: 0, duration: 0))
-        .toJson()));
+    r.ws.add(jsonEncode(HelloMessage(secret: secret, user: HelloUser(name: name)).toJson()));
     await r.next('welcome');
     return r;
   }
@@ -135,8 +130,8 @@ void main() {
           .cast<Map>()
           .firstWhere((m) => m['name'] == 'host'))['id'];
       // Anyone may sit the host out; its player unloads, the server serves on.
-      friend.ws.add(jsonEncode(OutIntent(target: '$hostId', out: true).toJson()));
-      await _poll(() => session.coordinator.selfOut.peek(), 'host sat out');
+      friend.ws.add(jsonEncode(PresenceIntent(target: '$hostId', state: Presence.away).toJson()));
+      await _poll(() => session.coordinator.selfAway.peek(), 'host sat out');
 
       // The host nudges the friend: only the friend hears it.
       final friendId = ((state['members'] as List)