Restructure step 5: one media pipeline on a player host

- PlayerHost port (MediaKitHost): players by role, pins, hwdec policy,
  render probe, log routing, disposal order; the only media_kit user.
- MediaPipeline: plan (one pure switch over state, origin, caps,
  presence) -> resolve -> open (prepared player promoted, else fresh)
  -> ready; commands queued during a switch apply once (P15); viewers
  play the buffer and wait for a dropped source without seek loops (D6).
- Every web page resolved by our yt-dlp into an EDL with headers and
  cookies (danmaku excluded); mpv's ytdl_hook no longer used (ytdl=no).
- D11: sign-in errors say which cookies were used and whether they hold
  any for the site.
- Relay: 256 KiB slices read as they fill, superseded pulls released
  (seek-to-play ~4.2 s -> ~2.8 s, P29); own Jellyfin relay channel only
  when someone needs it (P28).
- STARSYNC_SMOKE run and a Windows CI smoke step after the build.

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

apps/desktop/lib/src/platform/smoke.dart

@@ -0,0 +1,70 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart' show debugPrint;
+
+import '../app/app_graph.dart';
+import 'app_shutdown.dart';
+import 'ports/player_host.dart';
+
+// The close smoke run (CI, `tool/smoke_windows.ps1` / `tool/smoke_linux.sh`):
+// env `STARSYNC_SMOKE=<media file>` makes the app, once up, open that file
+// in a session player with a video output through the media pipeline,
+// play a second and close the normal way ([shutDownApp]). Progress goes to
+// `STARSYNC_SMOKE_OUT` (one line per step: `start`, `opened <ms> render=<mode>`,
+// `closing`, or `skip <why>` / `fail <why>`), so the script can tell a
+// runner that cannot show a window (skip) from a close that hangs or
+// leaves a process behind (fail).
+
+/// The smoke run's media, or null for a normal start.
+String? get smokeMedia {
+  final m = Platform.environment['STARSYNC_SMOKE'];
+  return m == null || m.isEmpty ? null : m;
+}
+
+Future<void> runSmoke(AppGraph graph, String media) async {
+  final outPath = Platform.environment['STARSYNC_SMOKE_OUT'];
+  void step(String line) {
+    debugPrint('smoke: $line'); // into the app log too
+    if (outPath == null || outPath.isEmpty) return;
+    try {
+      File(outPath).writeAsStringSync('$line\n', mode: FileMode.append, flush: true);
+    } catch (_) {}
+  }
+
+  step('start');
+  final sw = Stopwatch()..start();
+  final pipe = graph.coordinator.media;
+  try {
+    if (!await File(media).exists()) {
+      step('fail no media file at $media');
+    } else {
+      await pipe.start();
+      await pipe.playback.setVolume(0);
+      // The video output the session view would make: the render path
+      // (EGL / ANGLE) is what breaks on some machines.
+      final video = pipe.playback.video;
+      await video.ready.timeout(const Duration(seconds: 15));
+      final ok = await pipe.open(media);
+      final end = DateTime.now().add(const Duration(seconds: 20));
+      while (ok && pipe.playback.durationSecs <= 0 && DateTime.now().isBefore(end)) {
+        await Future.delayed(const Duration(milliseconds: 100));
+      }
+      if (pipe.playback.durationSecs <= 0) {
+        step('fail the media did not open in 20 s');
+      } else {
+        await pipe.playback.player.play();
+        await Future.delayed(const Duration(seconds: 1));
+        final mode = graph.ports.players.renderMode.peek();
+        step('opened ${sw.elapsedMilliseconds} render=${mode.name}');
+        if (mode == RenderMode.unknown) await Future.delayed(const Duration(seconds: 2));
+      }
+    }
+  } on TimeoutException {
+    step('skip the video output did not come up (no display for this runner?)');
+  } catch (e) {
+    step('fail $e');
+  }
+  step('closing');
+  await shutDownApp();
+}