v0.20.0 — music-only source audio for striped-LTC files

Hear only the music from a file that has timecode striped on it.

When an imported audio file carries LTC (linear timecode) on its own channel, OnlyCue now auto-mutes that channel so you hear only the music — the surviving music channel is centered to both ears — and the waveform draws the music track only instead of the timecode buzz, with a small badge showing the detected start timecode.

• Auto music-only playback — the detected LTC channel is muted by default on both the plain player and the LTC-output routing path; the music is centered to both ears.
• Music-only waveform + LTC badge — the preview shows only the music channel's peaks (no LTC buzz) and a badge with the detected start timecode.
• Per-clip Source audio toggle — Music only / Original (with timecode), in the Edit Media sheet and the clip's right-click menu, distinct from the existing LTC output mute. The choice is remembered in the project (.cuelist schema v19, with migration).
• Inert without LTC — files with no striped LTC play and render exactly as before.

Builds on v0.19.0's timecode-from-imported-audio scan (#712 / #713). Implements #715.

Note: the real-time channel muting is verified manually (the pure channel-mix and the persistence/migration are covered by unit tests; the app's UI-automation runner is currently unavailable for end-to-end capture).