feat(audio): mix-bus soft peak limiter

Replace the mixer's per-sample hard clamp with a lossless i32 bus sum fed
through a feed-forward soft limiter (instant attack, ~120ms release). Below
the ceiling it's transparent and sample-exact; loud multi-peer moments are
ridden down to the ceiling instead of shattering into hard-clip distortion.
State carries across frames so a sustained-loud stretch doesn't re-attack
every 20ms frame. The master output gain now applies inside the limiter so a
boost past the ceiling is limited too.

mix_frames now returns the lossless i32 sum (saturation responsibility moved
to the limiter); its tests assert losslessness, and the new limiter module
carries the saturation/transparency/release guarantees.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/audio/mod.rs

@@ -53,5 +53,6 @@ pub trait AudioBackend: Send + Sync {
 
 pub mod echo_cancel;
 pub mod gate;
+pub mod limiter;
 pub mod pipewire_impl;
 pub mod pw_cli;