instrument: one VELOCITY deck for all three velocity curves, bipolar and off by default for pitch and filter

Payload v12 appends the new velocity->pitch curve and folds the retired filter velAmount into its now-bipolar curve, so pre-v12 projects reopen sounding identical. Preview button takes a drawn play triangle.

src/core/instrument/engine/voice.h

@@ -48,6 +48,14 @@ inline double keyTrackedRatio(int note, int rootNote, double keyTrack) {
     return std::pow(2.0, semis / 12.0);
 }
 
+// 2^(curve(velocity) * kVelocityPitchRangeSemitones / 12): the velocity->pitch transpose, which
+// the voice folds into baseRatio_ once at note-on. A curve flat at 0 — the default — yields
+// EXACTLY 1.0 at every velocity and skips the pow, so an undrawn curve transposes nothing.
+inline double velocityPitchRatio(const VelocityCurve& curve, int velocity) {
+    const double semis = curve.eval(static_cast<double>(velocity)) * kVelocityPitchRangeSemitones;
+    return (semis == 0.0) ? 1.0 : std::pow(2.0, semis / 12.0);
+}
+
 // One octave expressed in the cutoff control's normalized domain, read out of the filter
 // module's OWN inverse rather than re-derived from its endpoints — the log law belongs to
 // filter_params, and a second copy here could drift from it. Evaluated at note-on only.
@@ -561,7 +569,8 @@ private:
     bool releasing_ = false;
     int note_ = 0;
     double velocityGain_ = 1.0;
-    double baseRatio_ = 1.0;     // 2^((note-root)/12): the un-modulated repitch ratio
+    double baseRatio_ = 1.0;     // key-tracked repitch ratio, with velocity->pitch folded in
+    double velPitchRatio_ = 1.0; // the velocity->pitch factor alone; retune re-applies it
     double ratio_ = 1.0;         // fractional source frames advanced per output frame (this frame)
     double readPos_ = 0.0;       // fractional frame index into the sample
     const SampleData* sample_ = nullptr;
@@ -593,7 +602,7 @@ private:
     double filterRate_ = 0.0;
     double filterCutoffNorm_ = 1.0;
     double filterModAmount_ = 0.0;
-    double filterVelOffset_ = 0.0;   // velAmount * velocityCurve.eval(velocity), fixed per note
+    double filterVelOffset_ = 0.0;   // velocityCurve.eval(velocity), fixed per note
     double filterKeyTrack_ = 0.0;
     instrument::engine::filter::FilterSettings filterSettings_{};  // the note's tone controls
     float filterBaseCutoff_ = 1.0f;    // cutoff before the envelope, clamped