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/server/test/jellyfin_relay_test.dart

@@ -11,6 +11,7 @@ const _serverId = 'fakeserver01';
 const _item = '0123456789abcdef0123456789abcdef';
 const _source = 'fedcba9876543210fedcba9876543210';
 const _slowItem = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
+const _nextItem = 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb';
 const _token = 'JF-SECRET-TOKEN';
 
 const _cred = JellyfinCredential(
@@ -368,6 +369,28 @@ void main() {
       expect(jf.uris, isEmpty, reason: 'nothing reached Jellyfin');
     });
 
+    test('Up next first item on the login: relayNext, and /j serves it too', () async {
+      await setUpRef(_item);
+      final a = clients.first;
+      expect(a.lastState!.containsKey('relayNext'), isFalse);
+      expect((await relayGet(token, _nextItem)).statusCode, 403);
+      a.send(const QueueIntent.add(QueueItem(
+              media: HelloMedia(
+                  name: 'Next',
+                  size: 0,
+                  duration: 0,
+                  jellyfin: JellyfinRef(serverId: _serverId, itemId: _nextItem, mediaSourceId: _source)),
+              how: QueueHow.jellyfin,
+              title: 'Next'))
+          .toJson());
+      await _until(() => a.lastState?['relayNext'] == true, 'relayNext');
+      final res = await relayGet(token, _nextItem, range: 'bytes=0-99');
+      expect(res.statusCode, 206);
+      expect(await readAll(res), bytes.sublist(0, 100));
+      expect((await relayGet(token, _item, range: 'bytes=0-9')).statusCode, 206, reason: 'the current one still');
+      expect((await relayGet(token, _slowItem)).statusCode, 403, reason: 'nothing else');
+    });
+
     test('non-Jellyfin media: 404', () async {
       await startServer(cred());
       final (a, w) = await join('alice');