Protocol 14: a warm relay preloads relayed Up next items for everyone

The server keeps one small relay (32 MiB ahead, 8 behind) for Up next's
first relayed item, keyed by its queue id at a URL that stays the same
after the swap: fetched by the server for server-fetch links, fed by
the adder's app for through-me links and files, by a member with an
account for member-passed Jellyfin items. Every app preloads from it;
the swap promotes it with its buffer. Dropped when the item stops
being first or its adder leaves. Measured switch: ~1.6 s cold, 20-40 ms
preloaded. ROADMAP gains the research item on server-side fetching of
member-resolved URLs.

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

packages/server/test/session_test.dart

@@ -64,8 +64,8 @@ void main() {
   });
 
   group('core welcome + proto', () {
-    test('proto 12: avatars by hash, profile intent', () {
-      expect(RelayConstants.protoVersion, 13);
+    test('proto 14: avatars by hash, profile intent', () {
+      expect(RelayConstants.protoVersion, 14);
       const v = MemberView(id: 'm1', name: 'a', ready: false, inSet: false, buffered: 0, stale: false);
       expect(v.toJson().containsKey('avatarHash'), isFalse);
       expect(v.toJson().containsKey('avatar'), isFalse);