Re-anchor the A/V clock for each stream

The master clock was anchored once to the first stream's audio PTS, but
every stream starts its own PTS timeline (~90000). Subsequent streams were
paced against the stale anchor, so their targets were all in the past and
the video free-ran to catch up (the reported speed-up / poor fps). Reset
the clock anchor when each stream starts.

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

@@ -200,6 +200,12 @@ public final class PlayerApp extends Application {
             try (UmdIsoReader iso = new UmdIsoReader(Path.of(isoPath));
                  PlaybackEngine engine = PlaybackEngine.open(iso, streamName, 0)) {
 
+                // Each stream has its own PTS timeline, so re-anchor the master
+                // clock to this stream's first audio chunk.
+                clockStartPts = -1;
+                clockStartNanos = -1;
+                clockOffsetNanos = 0;
+
                 Thread audio = startAudio(engine);
                 awaitClock();
                 long basePts = clockStartPts;