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/player_settings_test.dart

@@ -272,8 +272,8 @@ void main() {
 
     testWidgets('the state\'s host flag crowns that member for everyone', (tester) async {
       online(const [
-        MemberView(id: 'm1', name: 'Hana', ready: true, inSet: true, buffered: 30, stale: false, host: true),
-        MemberView(id: 'm2', name: 'Bob', ready: false, inSet: false, buffered: 0, stale: false),
+        MemberView(id: 'm1', name: 'Hana', ready: true, inSet: true, buffered: 30, host: true),
+        MemberView(id: 'm2', name: 'Bob', ready: false, inSet: false, buffered: 0),
       ]);
       final avatars = {for (final a in partyAvatars(coord)) a.name: a};
       expect(avatars['Hana']!.host, isTrue);
@@ -292,7 +292,7 @@ void main() {
     });
 
     testWidgets('hosting: Copy invite address lists this machine\'s addresses', (tester) async {
-      online(const [MemberView(id: 'm1', name: 'Me', ready: false, inSet: false, buffered: 0, stale: false)]);
+      online(const [MemberView(id: 'm1', name: 'Me', ready: false, inSet: false, buffered: 0)]);
       coord.hostFile = (_) async => null;
       SessionState.instance.lastForm.hostPort = '8080';
       bigWindow(tester);