Rework presets + add Flanger/Tremolo/Overdrive, per-preset gate, vocoder carrier Hz

- New effects: Overdrive (soft atan saturation), Tremolo (LFO AM),
  Flanger (modulated delay + feedback)
- Vocoder carrier is now parametrizable (carrier_hz) and CarrierKind::Input
  is available again; internal makeup raised so no giant external gains
- Per-preset gate_db: Demon/Ghost/etc. gate room noise; user gate wins
- Preset rework per review: Girl gets chorus+overdrive shimmer, Boy
  neutral formant + compressor, Demon less dirty (overdrive, gate, tremolo),
  Robot/Cyborg differentiated, Alien ring-mod 140Hz + flanger, Vader gets
  overdrive growl, Ghost tremolo, Walkie-Talkie bitcrush moved last,
  Growl compressor, Chipmunk drops bitcrush
- Gains normalized to moderate values (no more 6x clip factories)

src/config.rs

@@ -55,6 +55,7 @@ impl FileConfig {
                         formant_ratio: c.formant.unwrap_or(1.0),
                         effects: c.effects.clone().unwrap_or_default(),
                         gain: 1.0,
+                        gate_db: c.gate_db.unwrap_or(-100.0),
                     })
                     .collect()
             })