Harden playback/cache path for malformed media and concurrent timeline updates

- Invalidate timeline cache on ApplyJsonDiff() clip insert (remove affected frame range).
  - Add lock in Timeline::ClearAllCache() for safe concurrent access.
  - Make VideoCacheThread cross-thread state safe (atomics + seek-state mutex).
  - Lock CacheMemory::Contains() to avoid races.
  - Handle malformed audio streams in FFmpegReader by disabling invalid audio and continuing video-only.
  - Add FPS/timebase safety fallbacks in FFmpeg frame/PTS math.
  - Guard Frame::GetSamplesPerFrame() against invalid inputs.
  - Add/adjust regression tests for cache invalidation and invalid rate handling.

src/Timeline.cpp

@@ -1461,6 +1461,11 @@ void Timeline::apply_json_to_clips(Json::Value change) {
 		// Add clip to timeline
 		AddClip(clip);
 
+		// Calculate start and end frames that this impacts, and remove those frames from the cache
+		int64_t new_starting_frame = (clip->Position() * info.fps.ToDouble()) + 1;
+		int64_t new_ending_frame = ((clip->Position() + clip->Duration()) * info.fps.ToDouble()) + 1;
+		final_cache->Remove(new_starting_frame - 8, new_ending_frame + 8);
+
 	} else if (change_type == "update") {
 
 		// Update existing clip
@@ -1747,6 +1752,8 @@ void Timeline::apply_json_to_timeline(Json::Value change) {
 
 // Clear all caches
 void Timeline::ClearAllCache(bool deep) {
+	// Get lock (prevent getting frames while this happens)
+	const std::lock_guard<std::recursive_mutex> guard(getFrameMutex);
 
 	// Clear primary cache
 	if (final_cache) {