Additive engine: 5 presets across 3 tiers (Spectral Basics, Formant Play,
Harmonic Sculptor, Phase Explorer, Full Spectrum). FM engine: 5 presets
across 3 tiers (Simple FM, Bell Tones, Matrix Explorer, Feedback Machine,
Full Matrix).
Engine-aware preset system: applyPreset() dispatches to C15 groupOverrides
or new flat engineParamOverrides depending on active engine. Preset selector
dropdown rebuilds on engine switch. URL ?preset param searches all engines.
