diag(flutter): instrument playTracks + cache appdir; fix CI

CI fixes:
- artist_detail_screen.dart: drop unnecessary foundation import (debugPrint
  comes from material) and unused metadata_prefetcher import.

Playback timing visibility (so we can stop guessing where the lag
lives):
- playTracks now logs serverUrl / token / configure / setQueue /
  play() returned, each stage in milliseconds. The next time you
  tap play, we'll see exactly where the seconds go.
- setQueueFromTracks adds two more measurements: total source-build
  time across all tracks, and setAudioSources duration.

Small concrete win:
- audio_handler caches the application cache dir path on first use
  (already cached in _maybeRegisterStreamCache; now also used in
  _buildAudioSource for the LockCachingAudioSource path). One less
  platform channel hit per track on cache-miss queue builds.

Once we see real numbers we can decide whether the fix is to build
sources lazily (initial source first → play → background-add the
rest), pre-warm the audio handler at app start so playTracks skips
serverUrl + token reads entirely, or something else.

flutter_client/lib/player/player_provider.dart

@@ -53,8 +53,22 @@ class PlayerActions {
   final Ref _ref;
 
   Future<void> playTracks(List<TrackRef> tracks, {int initialIndex = 0}) async {
+    // Stage-by-stage timing so we can see exactly where the
+    // "tap → audio" delay lives. Look for "playTracks: ..." lines
+    // in the next log capture; each stage label is followed by its
+    // elapsed millis since the previous stage.
+    final sw = Stopwatch()..start();
+    int lap() {
+      final ms = sw.elapsedMilliseconds;
+      sw.reset();
+      sw.start();
+      return ms;
+    }
+
     final url   = await _ref.read(serverUrlProvider.future);
+    debugPrint('playTracks: serverUrl ${lap()}ms');
     final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
+    debugPrint('playTracks: token ${lap()}ms');
     final cache = _ref.read(albumCoverCacheProvider);
     final audioCache = _ref.read(audioCacheManagerProvider);
     final h = _ref.read(audioHandlerProvider)
@@ -64,8 +78,11 @@ class PlayerActions {
         coverCache: cache,
         audioCacheManager: audioCache,
       );
+    debugPrint('playTracks: configure ${lap()}ms');
     await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
+    debugPrint('playTracks: setQueue (${tracks.length} tracks) ${lap()}ms');
     await h.play();
+    debugPrint('playTracks: play() returned ${lap()}ms');
   }
 
   Future<void> playNext(TrackRef track) async {