Pro built-in voice stages #10

Rewrite the built-in DSP to plugin quality:

• Gate: voice-keyed (opens on speech, not keyboard clicks), level mode, hysteresis, hold, key high-pass, small lookahead, linked stereo.
• EQ: any number of bands (max 16), low/high pass at 12/24/48 dB/oct, notch, double-precision state, click-free changes.
• Compressor: soft knee, peak/RMS detector, log-domain smoothing, parallel mix.
• Leveler (new): ITU-R BS.1770 loudness target, moves only while speaking.
• De-esser: split-band and wideband.
• Limiter: true peak (see the limiter bug).
• Pitch shift: formant-preserving phase vocoder, independent formant shift.
• Reverb: FDN with modulation and pre-delay; chorus/delay with flat buffers, slewed delay time, high-cut feedback.

Each stage has behavior tests, per-block CPU numbers and an allocation check.