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>

tool/relay_e2e.dart

@@ -35,10 +35,9 @@ Future<void> main(List<String> argv) async {
     ws.add(jsonEncode(HelloMessage(
             proto: RelayConstants.protoVersion,
             secret: '',
-            user: HelloUser(name: name),
-            media: const HelloMedia(name: '', size: 0, duration: 0))
+            user: HelloUser(name: name))
         .toJson()));
-    // Stay alive like a real client (silent members get dropped).
+    // Stay alive like a real client (a silent member goes gone).
     Timer.periodic(const Duration(seconds: 1), (_) => ws.add(jsonEncode({'t': 'report', 'position': 0, 'buffered': 0, 'eventId': 0})));
     return (ws, await welcome.future);
   }