A fresh player for every media change (the frozen frame after a swap needed Ctrl+R); promotion and fresh players share one hand-over

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

apps/desktop/lib/src/session/switch_trace.dart

@@ -11,6 +11,8 @@ class SwitchTrace {
   static const state = 'state';
   static const resolved = 'resolved';
   static const open = 'open';
+  /// A fresh player is up for it (no preload held it).
+  static const player = 'player';
   static const opened = 'opened';
   static const preloaded = 'preloaded';
   static const buffered = 'buffered';