Protocol 13: Up next preloads, so switching to it is instant

Every app gets the first Up next item ready itself (own Jellyfin plan,
direct links, guests via /j when state.relayNext says the server's
login can pass it on) and opens it in a hidden, muted, paused second
player; the swap promotes that player instead of opening cold, so its
buffer counts at once. Reports go out on open and when the buffer
reaches the gate's need. SwitchTrace times each phase. Measured in the
e2e harness: 1.5-3 s before, 35-103 ms with a preload. Relayed items
still open cold for everyone but their source (ROADMAP).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/test/jellyfin_ref_test.dart

@@ -66,13 +66,16 @@ void main() {
           MemberView(id: 'm2', name: 'b', ready: false, inSet: false, buffered: 0, stale: false),
         ],
         relay: true,
+        relayNext: true,
       );
       final json = wire(state.toJson());
       expect(json['relay'], true);
+      expect(json['relayNext'], true);
       expect((json['members'] as List)[0], containsPair('jf', {'abc123': false}));
       expect((json['members'] as List)[1], isNot(contains('jf')));
       final back = StateBroadcast.fromJson(json);
       expect(back.relay, isTrue);
+      expect(back.relayNext, isTrue);
       expect(back.members[0].jf, {'abc123': false});
       expect(back.members[1].jf, isEmpty);
       expect(HelloMedia.fromJson(back.media!).jellyfin, ref);
@@ -80,7 +83,9 @@ void main() {
       final plain = StateBroadcast(
           eventId: 1, transport: 'held', position: 0, serverTime: DateTime.utc(2026), mediaId: 1, members: const []);
       expect(plain.toJson().containsKey('relay'), isFalse);
+      expect(plain.toJson().containsKey('relayNext'), isFalse);
       expect(StateBroadcast.fromJson(wire(plain.toJson())).relay, isFalse);
+      expect(StateBroadcast.fromJson(wire(plain.toJson())).relayNext, isFalse);
     });
 
     test('welcome relayUrl round-trips, omitted when null', () {