MKV Muxing Batch GUI 2.9.0

Added

• Add an enabled-by-default output guard that verifies every completed remux
  contains at least one audio track before accepting it or replacing a source.
• Add M2TS to the selectable video input extensions.
• Add regression coverage for Windows UNC source and destination paths.

Changed

• Allow multiple audio and subtitle tracks to be marked Default and Forced in
  both the main track editors and queued jobs.
• Pass MKVToolNix executable, probe, and response-file arguments directly to
  the process API so spaces, quotes, ampersands, and network paths remain
  literal rather than being interpreted by a shell.

Fixed

• Keep valid dropped videos when another selected file is not recognized as a
  video instead of clearing the complete selection.
• Report failed media probes per file and always close the loading dialog.
• Use portable temporary overwrite filenames without trailing spaces.
• Calculate CRC from the exact completed output path, report CRC failures, and
  avoid renaming partial outputs after a failed job.
• Finish failed queue jobs even when MKVToolNix exits without writing the log
  marker previously expected by the progress reader.
• Initialize all fields on existing-track data objects and avoid sharing a Qt
  table instance through a mutable constructor default.