Add pause/resume (Space, P, and media Play/Pause keys)

Pausing stops the audio line and blocks both playback threads; the A/V
clock is re-anchored on resume so video and audio continue exactly where
they left off without fast-forwarding.

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

@@ -55,6 +55,11 @@ public final class PlayerApp extends Application {
     private volatile long clockStartPts = -1;
     private volatile long clockStartNanos = -1;
 
+    private final Object pauseLock = new Object();
+    private volatile boolean paused = false;
+    private volatile long clockOffsetNanos = 0;
+    private long pauseStartNanos = 0;
+
     @Override
     public void start(Stage stage) {
         // Gradle's --args splits on whitespace; rejoin so paths with spaces work.
@@ -99,6 +104,14 @@ public final class PlayerApp extends Application {
     }
 
     private void handleKey(KeyCode code) {
+        switch (code) {
+            case SPACE, P, PLAY, PAUSE -> {
+                togglePause();
+                return;
+            }
+            default -> { }
+        }
+
         Controller c = controller;
         if (c == null) {
             return;
@@ -190,7 +203,8 @@ public final class PlayerApp extends Application {
                             frame.vPlane(), frame.vStride());
                     blitVideo(argb, frame.width(), frame.height());
 
-                    long targetNanos = baseNanos + (frame.pts() - basePts) * 1_000_000_000L / PlaybackEngine.MPEG_TIMESTAMP;
+                    checkPaused();
+                    long targetNanos = baseNanos + clockOffsetNanos + (frame.pts() - basePts) * 1_000_000_000L / PlaybackEngine.MPEG_TIMESTAMP;
                     sleepUntil(targetNanos);
                 }
 
@@ -218,7 +232,8 @@ public final class PlayerApp extends Application {
                         clockStartPts = chunk.pts();
                         clockStartNanos = System.nanoTime();
                     }
-                    long targetNanos = clockStartNanos
+                    checkPaused(line);
+                    long targetNanos = clockStartNanos + clockOffsetNanos
                             + (chunk.pts() - clockStartPts) * 1_000_000_000L / PlaybackEngine.MPEG_TIMESTAMP;
                     sleepUntil(targetNanos);
                     line.write(chunk.pcm(), 0, chunk.pcm().length);
@@ -267,6 +282,54 @@ public final class PlayerApp extends Application {
         }
     }
 
+    private void togglePause() {
+        synchronized (pauseLock) {
+            if (!paused) {
+                paused = true;
+                pauseStartNanos = System.nanoTime();
+            } else {
+                clockOffsetNanos += System.nanoTime() - pauseStartNanos;
+                paused = false;
+                pauseLock.notifyAll();
+            }
+        }
+    }
+
+    /** Blocks while playback is paused. */
+    private void checkPaused() {
+        synchronized (pauseLock) {
+            while (paused) {
+                try {
+                    pauseLock.wait();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    return;
+                }
+            }
+        }
+    }
+
+    /** Blocks while playback is paused, stopping/starting the audio line. */
+    private void checkPaused(SourceDataLine line) {
+        synchronized (pauseLock) {
+            boolean wasPaused = paused;
+            while (paused) {
+                if (line != null && line.isRunning()) {
+                    line.stop();
+                }
+                try {
+                    pauseLock.wait(100);
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    return;
+                }
+            }
+            if (wasPaused && line != null && !line.isRunning()) {
+                line.start();
+            }
+        }
+    }
+
     private void blitVideo(int[] argb, int width, int height) {
         WritableImage image = new WritableImage(width, height);
         image.getPixelWriter().setPixels(0, 0, width, height, PixelFormat.getIntArgbInstance(), argb, 0, width);