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/play_params.h

@@ -110,31 +110,32 @@ struct PitchEnvParams {
 // normalized control positions verbatim rather than a parallel set, so no control range is
 // re-derived here; `filter_params.h` owns every law that maps them to Hz/Q/depth.
 //
-// The three modulation depths below land in that same normalized cutoff domain and sum
-// before a single clamp; all three are zero/neutral by default.
+// The modulation depths below land in that same normalized cutoff domain and sum before a
+// single clamp; all are zero/neutral by default.
 struct FilterParams {
     bool enabled = false;
     instrument::engine::filter::FilterSettings settings;
     double modAmount = 0.0;   // bipolar [-1,+1], envelope -> cutoff
-    double velAmount = 0.0;   // bipolar [-1,+1], velocity -> cutoff
     double keyTrack  = 0.0;   // octaves of cutoff per octave of (note - root)
     // The filter envelope takes the same shape the amp does under the active play mode:
     // AHDSR in Gate, AHD in Trigger. Both are stored, so a mode flip cannot lose either
     // mode's dialled values (see core/instrument/CLAUDE.md).
     AdsrParams env;           // Gate: the same staged AHDSR the amp runs; frames
     AhdParams  trigEnv;       // Trigger: the same staged AHD the amp runs; frames
-    // Shapes velocity before velAmount scales it. Linear rather than the amp's flat() default
-    // because a flat curve under a depth control would make every velocity the same offset;
-    // the no-op at rest is velAmount == 0, not the curve. NOTE: this default only governs a
-    // FRESH FilterParams — the shared codec's corrupt/truncated-point-list repair
-    // (VelocityCurve::fromPoints, used for both this curve and the amp's) still degrades to
-    // flat() regardless, since that repair has no curve-specific fallback.
-    VelocityCurve velocityCurve = VelocityCurve::linear();
+    // Velocity -> cutoff, in the normalized cutoff domain. BIPOLAR, so the curve is both the
+    // shape and the amount — there is no separate depth knob behind it (the retired velAmount
+    // was exactly that, and a signed depth multiplying a signed curve made the sign unreadable).
+    VelocityCurve velocityCurve = VelocityCurve::zero();
 };
 
+// Full-scale of the velocity->pitch curve: y = +/-1 transposes by this many semitones. Shared
+// with the pitch envelope's own depth throw so the two pitch modulators speak one range.
+inline constexpr double kVelocityPitchRangeSemitones = 24.0;
+
 // Bundle a voice reads at start(). Defaults reproduce the bare engine (Gate, hold-0 AHDSR,
-// Varispeed, pitch envelope off, filter off) — core regression tests rely on this; the
-// Preserve product default is layered on at (de)serialization, see kDefaultPitchEngine.
+// Varispeed, pitch envelope off, filter off, no velocity->pitch) — core regression tests rely
+// on this; the Preserve product default is layered on at (de)serialization, see
+// kDefaultPitchEngine.
 struct PlayParams {
     PlayMode       playMode = PlayMode::Gate;
     AdsrParams     adsr;      // Gate amp
@@ -142,6 +143,10 @@ struct PlayParams {
     AhdParams      trigAhd;   // Trigger amp
     PitchEngine    pitchEngine = PitchEngine::Varispeed;
     PitchEnvParams pitchEnv;
+    // Velocity -> pitch offset, scaled by kVelocityPitchRangeSemitones. Bipolar and flat at 0
+    // by default, so it transposes nothing until a curve is drawn. Folded into the voice's
+    // baseRatio_ at note-on — it is fixed for the note's lifetime, so it costs no per-frame work.
+    VelocityCurve  pitchVelocityCurve = VelocityCurve::zero();
     FilterParams   filter;
 };