Auto/Manual/Off tail handling for realtime track-tap capture. Auto scans
recorded PCM backward for last frame above -72 dB and truncates the wav
header-aware (8 s cap); Manual pads a fixed tail; Off is byte-identical.
wav_trim rejects WAVE_FORMAT_EXTENSIBLE with non-float SubFormat GUID.
Capture is now item + track only (master removed as a scope; still bypassed
as out-of-scope chain). Realtime records the selected track's own output via
per-track post-fader sends into a hidden temp track, fixing the silent file.
Timer-driven realtime capture behind ICaptureBackend (begin/tick/abort via
OnTimer, non-blocking). Records master into a hidden temp track, moved to the
bank non-destructively with idempotent restore across every terminal path.
Pure phase machine unit-tested. Track/item deferred; realtime is non-deterministic.