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/lib/src/jellyfin/session_media.dart

@@ -4,6 +4,7 @@ import 'package:flutter/foundation.dart';
 import 'package:sync_relay_core/core.dart';
 
 import '../screens/home/picked_media.dart';
+import '../session/party.dart' show isPresent;
 import 'accounts.dart';
 import 'format.dart';
 
@@ -333,13 +334,13 @@ Duration nextPromptStart(JellyfinItem? item, Duration runtime, {Duration? ending
 
 /// Who performs the auto-advance swap, so several counting clients do not
 /// swap twice: the lowest member id among members that play the item with
-/// their own account (`jf[serverId] == true`, not stale, not out). Only what every
+/// their own account (`jf[serverId] == true`, present: `in` or joining). Only what every
 /// client sees in `state` decides (who put the item up is not in it: a
 /// late joiner never saw that swap), so all clients agree.
 String? autoAdvanceSwapper(List<MemberView> members, String serverId) {
   final eligible = [
     for (final m in members)
-      if (!m.stale && !m.out && m.jf[serverId] == true) m
+      if (isPresent(m) && m.jf[serverId] == true) m
   ];
   if (eligible.isEmpty) return null;
   eligible.sort((a, b) => compareMemberIds(a.id, b.id));