WIP/experimental: render an AEC reference into VPIO (does NOT cancel yet — see real-hardware test)

[louis030195]

Gives VoiceProcessingIO the downlink reference its echo canceller needs. Today screenpipe's VPIO is input-only (output element disabled), so Apple's AEC has nothing to subtract and removes 0 dB (screenpipe/screenpipe#3938).

New optional MacosVoiceProcessingInputConfig.aec_reference (a shared mono-f32 ring). When set: enable + mute the output element, force it to mono f32, and install a realtime render callback that drains the ring into it as the AEC far-end reference. The engine feeds it the captured system-audio lane. None = unchanged input-only behavior.

Compiles clean on macOS. Not yet runtime-validated — needs a real Mac (speakers, no headphones, join a call, confirm the remote stops appearing under "me"). Engine-side wiring (feed system-audio into the ring + pass it to this config) is a paired screenpipe PR.

[louis030195]

⚠️ Real-hardware test: this bridge does not work yet (makes echo worse)

Built it into screenpipe-audio (cpal re-pinned here) and tested on a real Mac: opened a VPIO mic stream with the reference ring, played a speech clip through the speakers, fed the same clip into the ring, captured the mic, and compared echo correlation (mic↔far, wide-search) with the reference ON vs OFF.

• Native mic rate is 96 kHz; VPIO is a voice unit, so I forced the stream to 48 kHz.
• WITHOUT reference (baseline echo): corr ≈ 4.1
• WITH reference: corr ≈ 6.0 — echo went UP, not down.

Interpretation: the rendered reference is not being muted — kAUVoiceIOProperty_MuteOutput isn't taking effect, so the far-end I render into the output element is being played back through the speakers, adding more echo to the mic. So the bridge is currently counterproductive, and separately it's still unproven that VPIO's AEC consumes an app-injected reference at all (vs only audio it routes through its own playback path).

Status: experimental / does not cancel. Needs real CoreAudio debugging — verify MuteOutput scope/element actually mutes, confirm the reference reaches VPIO's AEC reference bus, and validate VPIO engages AEC at the chosen rate. Not mergeable as-is.