Decode on a single background thread; add intro skip key

PlaybackEngine now runs one daemon decode thread that routes packets into
bounded queues, while the video/audio presenter threads only consume and
pace. This removes the demuxer contention that caused stutter, bursts, and
the speed-up/catch-up behavior.

Press S (or media next) during playback to skip the current playlist and
advance the menu's intro chain quickly.

ui/src/main/java/dev/quoije/umd/ui/PlayerApp.java

@@ -64,6 +64,7 @@ public final class PlayerApp extends Application {
     private final WritableImage[] videoImages = new WritableImage[2];
     private final int[][] argbBuffers = new int[2][];
     private int videoBufferIndex = 0;
+    private volatile boolean skipRequested = false;
 
     @Override
     public void start(Stage stage) {
@@ -114,6 +115,10 @@ public final class PlayerApp extends Application {
                 togglePause();
                 return;
             }
+            case S, TRACK_NEXT -> {
+                skipRequested = true;
+                return;
+            }
             default -> { }
         }
 
@@ -202,6 +207,10 @@ public final class PlayerApp extends Application {
 
                 PlaybackEngine.VideoFrame frame;
                 while ((frame = engine.nextVideoFrame()) != null) {
+                    if (skipRequested) {
+                        skipRequested = false;
+                        break;
+                    }
                     int index = videoBufferIndex++ & 1;
                     int[] argb = argbBuffers[index];
                     int pixelCount = frame.width() * frame.height();