v1.2.0

v1.2.0 — Tape rewrite, AutoGain calibrate-and-lock, reverb 96 kHz fix

Headline changes since v1.1.3:

- Tape mode: replaced mu-law codec with linear buffer + SoftClip output.
  Fixes "loud overload sounds" reported when many grains stacked
  constructively in the compressed-domain buffer.
- AutoGain: 5 s calibration window, locks at -10 dBFS, no ratcheting.
  Earlier ratchet measured AutoGain's local output instead of the bus
  peak and gave a false sense of clip protection.
- Reverb: SR-scaled delays + Butterworth biquad LP keyed in Hz.  Fixes
  the 4 kHz resonance peak that appeared at 96 kHz sample rate.
- kMaxGrains 20 → 30 to match the original Beads "30 replay heads"
  spec.  CPU headroom from earlier optimizations (DTC grain cache, -O2,
  FastPowUnit, etc.) absorbs the bump cleanly on disting NT.
- Input chain: 6-pole Butterworth LP cascade replaces 4-pole.
- Grain engine: sub-block phase decoherence, read-offset wrap glitches,
  and decimated-mode feedback regen all fixed.
- New spectral test harness with FRA, THD, IR, and wow/flutter gates.

Validated by listening tests on hardware (disting NT) at 48 kHz and
96 kHz.