feat(lotus-denoise): quality — dry/wet attenuation floor, gate-after-ML, softer DFN

Track-B audio-quality changes to reduce the "robotic/underwater" artifact.

- Dry/wet attenuation floor (default 0.15 ≈ -16 dB) blends a little of the raw
  mic under the denoised signal so suppression can't fully collapse the noise
  floor between words (the main cause of the RNNoise "underwater"/pumping
  sound). Applied ONLY to the low-latency flat models (RNNoise/Speex); DTLN/DFN
  add algorithmic latency that would comb-filter an undelayed dry mix, so they
  rely on their own level instead. Tunable via `lotusDenoiseFloor`.
- Noise gate now runs AFTER the ML model, not before — gating the raw signal
  fed hard-zeroed frames into the model and tuned the threshold on pre-denoise
  levels.
- DeepFilterNet 3 noiseReductionLevel 80 -> 60: full strength was the main
  "over-processed" contributor; 60 keeps voice natural.

Defaults are conservative and tunable; final values are meant to be dialed in
with real-call A/B listening.

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

src/lotus/lotusDenoise.ts

@@ -73,6 +73,7 @@ export function startLotusDenoise(vm: CallViewModel): () => void {
       : "rnnoise";
 
   const rawThreshold = lotusParam("lotusGateThreshold");
+  const rawFloor = lotusParam("lotusDenoiseFloor");
   const config: LotusDenoiseConfig = {
     model,
     assetBase: safeAssetBase(lotusParam("lotusDenoiseBase")),
@@ -83,6 +84,13 @@ export function startLotusDenoise(vm: CallViewModel): () => void {
       rawThreshold !== null && Number.isFinite(Number(rawThreshold))
         ? Number(rawThreshold)
         : -45,
+    // Dry/wet attenuation floor. Default 0.15 (~-16 dB) tames the
+    // over-suppression "underwater"/pumping artifact; host can tune via
+    // `lotusDenoiseFloor` (0 = full suppression, no floor).
+    floor:
+      rawFloor !== null && Number.isFinite(Number(rawFloor))
+        ? Math.min(0.5, Math.max(0, Number(rawFloor)))
+        : 0.15,
   };
 
   const micOf = (room: LivekitRoom): LocalAudioTrack | undefined =>