fix(denoise): DeepFilterNet attenuation limit 60 -> 25 dB

At 60 dB the background is muted outright (measured -61 to -64 dB), so
each word drops into dead silence and listeners hear the mic cutting out
at word ends. On room-echo speech (RT60 ~0.4 s) with fan and keyboard
noise at SNR 15 dB, 25 dB scored best:

  limit  PESQ clean/fan/keyboard   noise removed
  60     3.35 / 2.33 / 2.04        ~62 dB
  40     3.41 / 2.45 / 2.21        32-40 dB
  25     3.59 / 2.72 / 2.38        25 dB
  20     3.63 / 2.66 / 2.25        20 dB

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/lotus/lotusDenoiseProcessor.ts

@@ -207,10 +207,12 @@ async function loadDfnCore(config: LotusDenoiseConfig): Promise<DfnCore> {
   );
   const core = new mod.DeepFilterNet3Core({
     sampleRate: 48_000,
-    // 60, not 80: full-strength suppression is the main source of the
-    // "over-processed" character; a lower level keeps voice natural while
-    // the dry/wet floor handles the noise tail.
-    noiseReductionLevel: 60,
+    // Attenuation limit in dB. At 60 the background is muted outright
+    // (measured -61 to -64 dB), so every word drops into dead silence and
+    // listeners hear the mic "cutting out" at word ends. 25 keeps a quiet
+    // background bed and scored best on room-echo speech with fan and
+    // keyboard noise (PESQ 2.72 / 2.38 vs 2.33 / 2.04 at 60).
+    noiseReductionLevel: 25,
     assetConfig: { cdnUrl: dfnBase },
   });
   await core.initialize();