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/jellyfin_session_test.dart

@@ -60,9 +60,15 @@ JellyfinMediaSource fixtureSource() => JellyfinPlaybackInfo.fromJson(
         jsonDecode(jellyfinFixture('playback_info')) as Map<String, dynamic>)
     .primarySource!;
 
-MemberView member(String id, String name, {Map<String, bool> jf = const {}, bool stale = false}) =>
+MemberView member(String id, String name, {Map<String, bool> jf = const {}, bool gone = false}) =>
     MemberView(
-        id: id, name: name, ready: true, inSet: true, buffered: 10, stale: stale, jf: jf);
+        id: id,
+        name: name,
+        ready: true,
+        inSet: true,
+        buffered: 10,
+        presence: gone ? Presence.gone : Presence.watching,
+        jf: jf);
 
 void main() {
   setUp(reports.clear);
@@ -371,7 +377,7 @@ void main() {
         member('m10', 'Ann', jf: {fixtureServerId: true}),
         member('m2', 'Bob', jf: {fixtureServerId: true}),
         member('m1', 'Guest', jf: {fixtureServerId: false}),
-        member('m0', 'Old', jf: {fixtureServerId: true}, stale: true),
+        member('m0', 'Old', jf: {fixtureServerId: true}, gone: true),
         member('m3', 'Fresh'),
       ];
       expect(autoAdvanceSwapper(members, fixtureServerId), 'm2');