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/session/party.dart

@@ -0,0 +1,27 @@
+import 'package:sync_relay_core/core.dart';
+
+// The app's reading of presence (PROTOCOL.md "Presence"): every count,
+// list and wait in the UI goes through these, on top of core's
+// [PresencePolicy] and [isAlone].
+
+/// What each presence counts for (the one table, shared with the server).
+const presencePolicy = PresencePolicy();
+
+/// Counted in "2/3 ready" and waited for by name: its presence counts for
+/// the ready counter and its player can open the media.
+bool countsForReady(MemberView m) => m.error.isEmpty && presencePolicy.countsFor(m.presence, Purpose.readyCounter);
+
+/// Connected and watching (`in` or `joining`): can do things for the
+/// party now (put up an Up next item, the auto-advance swap).
+bool isPresent(MemberView m) => m.presence == Presence.watching || m.presence == Presence.joining;
+
+/// Nobody but [selfId] is connected in [members] (D9, core's [isAlone]).
+bool aloneIn(List<MemberView> members, String selfId) =>
+    isAlone([for (final m in members) if (m.id != selfId) m.presence]);
+
+/// "3 min" / "40 s": how long since [since] (a `goneSince`), at [now].
+String sinceText(DateTime since, DateTime now) {
+  final d = now.difference(since);
+  if (d.inMinutes >= 1) return '${d.inMinutes} min';
+  return '${d.inSeconds.clamp(0, 59)} s';
+}