Protocol 11: split-track relay (YouTube video+audio), chunked upstream reads

Relayed media can carry 2-4 byte tracks (video, audio): yt-dlp picks a
split pair when no single-file format exists (h264 first), the source
channel serves each track, the server keeps a buffer per track (budget
split by size, 8 MiB floor) and viewers open an mpv edl:// of the track
URLs. Expired URLs re-resolve once for all tracks. Upstream GETs are cut
into <= http_chunk_size (10 MiB) ranges: YouTube throttles larger ones.
Live streams and DRM are refused; HLS/DASH pieces are a follow-up.

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

apps/desktop/test/relay_e2e_test.dart

@@ -123,7 +123,7 @@ void main() {
     expect(bob.relaying.peek(), isTrue);
     await playTogether([alice, bob]);
     final relay = server.hub.relay!;
-    expect(relay.buffer.bytesToViewers, greaterThan(0), reason: 'alice played through the relay');
+    expect(relay.buffers.single.bytesToViewers, greaterThan(0), reason: 'alice played through the relay');
     expect(relay.sourceToken, bob.sync.token);
 
     // The next swap retires bob's source and the buffer.
@@ -144,7 +144,7 @@ void main() {
       await _poll(() => stateMediaRelay(alice.serverState.peek()!)?.source == bob.sync.memberId, 'me state');
       expect(alice.serverState.peek()!.toJson().toString(), isNot(contains('SECRET')));
       await playTogether([alice, bob]);
-      expect(server.hub.relay!.buffer.bytesToViewers, greaterThan(0));
+      expect(server.hub.relay!.buffers.single.bytesToViewers, greaterThan(0));
 
       final before = hits.length;
       expect(await alice.swapPicked(LinkMedia(link, relay: LinkRelay.server)), isNull);