Add owned capture DSP and explicitly reviewed calibration

[rikkichy]

Reviewed replacement for #8, part 9/12. Original contribution: Zedwil / NyleGarcia.

Stack

Depends on #20; base is pr8-08-scenes-actions. Merge the parent first. These are ordered review slices, not independent cherry-picks.

Scope

• Validate and render per-capture low cut, gate, compressor, EQ, delay and optional mono using owned PipeWire filter processes.
• Preserve stereo and exact neutral bypass; a failed enabled chain silences sends instead of exposing unprocessed audio.
• Measure the raw selected capture in a cancellable worker, require explicit proposal acceptance, and reject stale hotplug identities. Never adjust hardware gain or phantom power.

Verification

Real private PipeWire/SWH PCM verified stereo EQ, mono, gate/compressor, bypass and failed-plugin silence. Actual GTK calibration remained responsive; cancel left settings untouched, explicit apply worked, and unplug/replug prevented stale application.

Runtime scenarios were exercised on the integrated stack. Hardware-facing tests do not imply physical USB/phantom-power certification.