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/delivery_test.dart

@@ -7,7 +7,7 @@ import 'package:starsync/src/session/delivery.dart';
 import 'package:starsync/src/shell/delivery_line.dart';
 
 MemberView _m(String id, String name, {double buffered = 30, bool host = false, int rate = 0}) =>
-    MemberView(id: id, name: name, ready: true, inSet: true, buffered: buffered, stale: false, host: host, rate: rate);
+    MemberView(id: id, name: name, ready: true, inSet: true, buffered: buffered, host: host, rate: rate);
 
 StateBroadcast _state(Map<String, Object?>? media, List<MemberView> members, {String transport = 'playing'}) =>
     StateBroadcast(
@@ -87,7 +87,7 @@ void main() {
   test('Jellyfin: own accounts direct, the rest via server', () {
     const ref = {'serverId': 'srv', 'itemId': '0123456789abcdef0123456789abcdef', 'title': 'Pilot'};
     MemberView jf(String id, String name, bool own) =>
-        MemberView(id: id, name: name, ready: true, inSet: true, buffered: 30, stale: false, jf: {'srv': own});
+        MemberView(id: id, name: name, ready: true, inSet: true, buffered: 30, jf: {'srv': own});
     final all = [jf('m1', 'Foxyz', true), jf('m2', 'Bob', true)];
     final media = {'name': 'Pilot', 'jellyfin': ref, 'relay': const MediaRelay(source: 'm1', live: true).toJson()};
     final d = deliveryOf(_state(media, all), selfId: 'm2')!;