[AlxcBTW]

Party voice on speakers echoed: the far member's voice went from the speakers into the microphone and back to the party. The engine now runs WebRTC's AEC3 (the pure-Rust port sonora, BSD-3-Clause) on the microphone before RNNoise and the gate.

• ADR 0040 records the choice and the rejected options (tonarino C++ bindings need meson/ninja or a system package, OS-level cancellers differ per platform, speex).
• Config::echo_cancellation (default on), Engine::set_echo_cancellation, control command voice_echo { enabled } (new command, old launchers never send it).
• The reference is the mixer's own output, released to the canceller when the device has taken it off its queue; the canceller is bypassed while nothing is played.
• Workspace rust-version 1.88 -> 1.91 (sonora MSRV; CI uses stable).
• Tests (offline, fixed seeds): canceller ERLE 44.7 dB, whole engine through Opus 50 dB (0.8 dB with it off), transparent with a silent reference, double talk.

Not verified: real speakers/microphones.

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