fix(wasm): rebase timeline anchor after pace-drop to prevent playback stall

The rAF display loop in rtp_demo.html dropped late frames without
rebasing wallStart/rtpStart, so every subsequent frame computed a
target far in the past and was also dropped — cascading until the
ring emptied and playback froze. Particularly visible with the AWS
CloudFront 150-frame fixture. Mirrors the native RTP receiver's
"runaway rebase" logic (pipeline_multi_threaded.cpp:445-452).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: osamu620

web/rtp_demo.html

@@ -1547,10 +1547,16 @@ <h1>HTJ2K RTP Replay — Browser WASM Demo</h1>
       const target = wallStart + totalPausedMs + dtMs;
       const period = entry.framePeriodMs || (1000 / 60);
 
-      // Pace-drop: too far behind?  Discard and re-check next entry.
+      // Pace-drop: too far behind?  Discard and rebase the timeline
+      // anchor so the next surviving frame is scheduled relative to
+      // "now" — without this, every subsequent frame stays behind the
+      // stale anchor and gets pace-dropped too, stalling playback.
+      // Mirrors the "runaway rebase" in the native RTP receiver.
       if (paceDropEnabled() && rafTimestamp - target > PACE_DROP_PERIODS * period) {
         ringPop();
         droppedByPace++;
+        wallStart = rafTimestamp;
+        rtpStart  = (_ringCount > 0) ? ringPeek().rtpTimestamp : rtpStart;
         continue;
       }