Make every preset as loud as 8-bit: per-preset output makeup gain

The vocoder/ring-mod/bandpass presets were 10-20 dB quieter than 8-bit.
Each preset now carries an output makeup gain normalized from tone
measurements; all presets peak around -1..-6 dB (was -9..-20).

src/config.rs

@@ -54,6 +54,7 @@ impl FileConfig {
                         pitch_semitones: c.pitch.unwrap_or(0.0),
                         formant_ratio: c.formant.unwrap_or(1.0),
                         effects: c.effects.clone().unwrap_or_default(),
+                        gain: 1.0,
                     })
                     .collect()
             })