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/state_diff_test.dart

@@ -130,22 +130,19 @@ void main() {
         name: 'a',
         ready: true,
         inSet: true,
-        buffered: 9,
-        stale: false);
+        buffered: 9);
     const catchingUp = MemberView(
         id: 'm2',
         name: 'b',
         ready: false,
         inSet: false,
-        buffered: 0,
-        stale: false);
+        buffered: 0);
     const unreadyInSet = MemberView(
         id: 'm3',
         name: 'c',
         ready: false,
         inSet: true,
-        buffered: 9,
-        stale: false);
+        buffered: 9);
 
     test('only in-set ready members may follow transport to playing', () {
       final state = _stateWithMembers([ready, catchingUp, unreadyInSet]);