- Prevent cross-category graph leakage (png -> txt -> docx) in ConversionGraph.FindBestPath and ReachableOutputs
- Distinguish Video and Audio in MediaConversionNegotiator (block mp3->gif, allow mp4->png frame extraction)
- Handle empty common output intersection in MainWindow without forcing .jpg fallback
- Expand PopularOutputs in ContextMenuRegistrar to support mp4, mp3, and xlsx
- Add comprehensive TDD unit tests (213 passing tests)
