feat: per-channel dry/wet mix (parallel compression) replacing bypass

Replace the per-channel bypass toggle with a smoothed dry/wet `mix` (0..100%,
default 100%). The blend is applied at the compressor output:
  out = delayed_input * ((1 - mix) + mix * wet_gain)
Dry and wet share the same delayed input, so it's phase-aligned (parallel
compression, no comb filtering). mix=0 is bit-identical to the old bypass.

The detector now runs even at mix 0, so the GR meter shows the wet gain
reduction regardless of mix, while the level/plot out trace reads the mixed
output. "Bands at 0% = simple full-band comp via All" still holds.

Update README + parameter docs (bypass -> mix).

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

src/editor/mod.rs

@@ -62,7 +62,8 @@ pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Opt
                 ui.add(widgets::ParamSlider::for_param(&p.release_ms, setter));
                 ui.label("Makeup");
                 ui.add(widgets::ParamSlider::for_param(&p.makeup_db, setter));
-                ui.add(widgets::ParamSlider::for_param(&p.bypass, setter));
+                ui.label("Mix");
+                ui.add(widgets::ParamSlider::for_param(&p.mix, setter));
             };
 
             // Resizable window; vertical scroll so every control stays reachable even when the