Read once: a link passed on is fetched from the site once, for the putter's player and the relay

Before, the member passing a page link on downloaded it twice, independently:
their mpv straight from the CDN and the relay's pulls again, with a
different format pick and without yt-dlp's cookies. The two drifted (the
putter played on while viewers stalled, or the reverse) and doubled the
connections the CDN saw. Now one SharedSource per track reads the site
once (resuming after cuts, backing off on 412/429/5xx and silence) into a
bounded buffer (64 MiB ahead, 16 behind) that serves both the putter's
mpv through a loopback endpoint and the server's pulls; the putter plays
the relay's own format pick, and the relay sends yt-dlp's cookies.
Up next's warm relay uses the same reader. Pieces tracks (HLS/DASH) keep
two fetches for now. Measured in the opt-in e2e: half the CDN traffic, no
412s under a lockout CDN, viewer seeks 0.29 s instead of 4 s there.

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

apps/desktop/test/relay_e2e_test.dart

@@ -159,12 +159,12 @@ class _BareSource {
   }
 }
 
-/// Serves one file with ranges; [requests] counts hits.
+/// Serves one file with ranges; [requests] lists hits (user agent, URL).
 Future<(HttpServer, List<String>)> _origin(String file) async {
   final s = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
   final requests = <String>[];
   s.listen((req) async {
-    requests.add(req.uri.toString());
+    requests.add('${req.headers.value('user-agent')} ${req.uri}');
     final size = await File(file).length();
     final r = resolveMediaRequest(rangeHeader: req.headers.value('range'), size: size, filename: 'v.mkv');
     req.response.statusCode = r.status;
@@ -274,6 +274,12 @@ void main() {
       expect(alice.serverState.peek()!.toJson().toString(), isNot(contains('SECRET')));
       await playTogether([alice, bob]);
       expect(server.hub.relay!.buffers.single.bytesToViewers, greaterThan(0));
+      // Read once: bob's player plays his shared reader's loopback, so
+      // every hit at the origin is that reader's (Dart's), none mpv's.
+      final one = bob.media.shared.currentFor(link);
+      expect(one, isNotNull);
+      expect(one!.bytesIn, greaterThan(0));
+      expect(hits.where((h) => !h.startsWith('Dart/')), isEmpty, reason: '$hits');
 
       final before = hits.length;
       expect(await alice.swapPicked(LinkMedia(link, relay: LinkRelay.server)), isNull);