With Enable Logging on, MPVBackend watches hwdec-current (property event
plus a single-property poll in the stats loop, since mpv does not always
emit an event for a silent fallback). When a file that was hardware
decoded drops to software, it logs a warning with codec/stream, playback
position, scene phase and time since the last change, thermal/low-power
state, PiP and the recent mpv log lines, and shows a toast asking to
export logs (deferred until the app is active again).
The background diagnostics heartbeat now includes the decoder state.
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