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/queue_switch_e2e_test.dart

@@ -343,6 +343,13 @@ void main() {
           final relay = stateMediaRelay(a.serverState.peek()!);
           expect(relay?.queueId, id, reason: 'the warm relay became the media\'s');
           expect(relay?.fetch, mode == LinkRelay.server);
+          if (mode == LinkRelay.me) {
+            // Read once: alice's preload and the warm relay read one shared
+            // reader, and the swap carried it on as the media's.
+            final one = a.media.shared.currentFor(linkUrl);
+            expect(one, isNotNull, reason: 'alice reads the link once');
+            expect(one!.bytesIn, greaterThan(0));
+          }
           if (preload) {
             for (final x in all.values) {
               expect(x.switchTrace.of(x.serverState.peek()!.mediaId), contains(SwitchTrace.preloaded),