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/app_keys_test.dart

@@ -43,7 +43,7 @@ void main() {
       position: 0,
       serverTime: DateTime.now().toUtc(),
       mediaId: 0,
-      members: [MemberView(id: 'a', name: 'Alex', ready: false, inSet: false, buffered: 0, stale: false)],
+      members: [MemberView(id: 'a', name: 'Alex', ready: false, inSet: false, buffered: 0)],
     );
     session.route.value = AppRoute.session;
     await tester.pumpAndSettle();