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>

packages/server/test/lifecycle_test.dart

@@ -1,5 +1,3 @@
-// restructure: rewrite in step 1 (Connection + Identity + Presence): a
-// socket close no longer removes the member (explicit `bye`, grace).
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -74,14 +72,22 @@ void main() {
       await server.stop();
     });
 
-    test('clean client close removes member within seconds, not timeout',
-        () async {
+    test('a clean close is not a leave: the member is gone at once, kept', () async {
       final client = await _ProbeClient.hello(server.boundPort, 'alice');
-      await _pollBool(
-          () => server.hub.engine.members.length == 1, 'join visible');
+      await _pollBool(() => server.hub.engine.members.length == 1, 'join visible');
       await client.ws.close();
-      await _pollBool(
-          () => server.hub.engine.members.isEmpty, 'member removed');
+      await _pollBool(() => server.hub.engine.members.values.single.presence == Presence.gone, 'gone');
+      expect(server.session.tokens, hasLength(1), reason: 'its token works through the grace');
+      expect(server.hub.sockets, isEmpty);
+    });
+
+    test('bye leaves at once: member, token and socket go', () async {
+      final client = await _ProbeClient.hello(server.boundPort, 'alice');
+      await _pollBool(() => server.hub.engine.members.length == 1, 'join visible');
+      client.ws.add(jsonEncode(const ByeMessage().toJson()));
+      final outcome = await client.closed.future.timeout(const Duration(seconds: 5));
+      expect(outcome, contains('code=${CloseCodes.left}'));
+      expect(server.hub.engine.members, isEmpty);
       expect(server.session.tokens, isEmpty);
       expect(server.hub.sockets, isEmpty);
     });