Tests cover:
- Default state (C15, 126 params, heatmap cells)
- Engine switcher UI (3 cards, active state)
- Switch to Additive (48 params, outputs bounded, training works)
- Switch to FM Matrix (55 params, outputs bounded, training works)
- Round-trip switching (C15→Add→C15, C15→FM→Add→C15)
- Warm-start weight preservation across resize
- EOC chain accessibility across engines
- SynthVisualizer visibility across engines
- No console errors during any switch
Also fixes: faustJsonToParamMeta now filters [hidden:1] params,
giving correct counts (48 additive, 55 FM) instead of including
freq/gate/_vel control inputs.
