Track observed OBS mix assignments through transcription

[RioPlay]

When OBS inputs are renamed or assigned to different buses during an authorized capture, retain each observed assignment alongside the transcript and show the latest configuration in a compact Mix details tab. Ordinary assignment updates keep audio capture continuous; invalid capture identity or metadata fails visibly.

This adds bounded native observation, version-2 routing frames, a private disk-backed metadata journal, and cancellation-safe ownership through transcription. Mix descriptions distinguish matching input assignments from isolated speakers or audible content, and disclose that observation timestamps are approximate. The desktop requires audio version 2 before Arm; command and enrollment protocols remain version 1.

Validation on the reviewed source:

• 873 affected desktop tests and 48 Tk tests passed; eleven UI captures inspected.
• All 55 native verification commands, the 25-command linked build, and headless refusal smoke passed.
• 488 source, dependency, build, test, and UI hash comparisons matched.
• Independent review and deterministic regressions cleared stop/drain, stale refresh, retirement/publication, and callback scheduling races.

All five desktop jobs passed for 813cce7 in source CI, including Windows and macOS builds/tests and Linux X11/Wayland tests plus package smoke checks. These checks use synthetic fixtures; live OBS/audio/load acceptance, the application entry point, export integration, and native distribution remain open. No plugin binary is published by this PR. See the active contract.