feat(mq_editor): partial spectrum viewer — synth+FFT power displayHEADmain

Adds a 200×100 canvas (left of the main spectrum overlay) that shows
the synthesised power spectrum of the selected partial at the time
under the mouse (or playhead).

Pipeline: synthesizeMQ → Hann window → FFT (2048-pt) → dB power bars.
- freqCurve times are shifted so the synthesis window is centred on t
- X-axis: log-frequency (same scale as main view)
- Y-axis: dB, normalised to peak of the synthesised frame
- Cache: {partialIndex, time} → avoids re-synthesis on mouse move;
  bypassed (force=true) from render() so param changes always redraw

handoff(Claude): partial spectrum viewer complete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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