S-VIEW-9: velocity->amp transfer curve (pure velocity_curve module + zones-payload v7 + Voice::start apply)

Default flat y=1 (R10-F1 Option A) replaces the linear velocity/127 at note-on — a deliberate, non-back-compat behavior change; v1-v6 blobs lift to the flat default.

src/vst/sampler_core.h

@@ -21,8 +21,9 @@
 #include <cstdint>
 #include <vector>
 
-#include "peaks.h"        // AudioSample (float)
-#include "pitch_shift.h"  // PitchShifter (S16 Preserve engine DSP core)
+#include "peaks.h"           // AudioSample (float)
+#include "pitch_shift.h"     // PitchShifter (S16 Preserve engine DSP core)
+#include "velocity_curve.h"  // VelocityCurve (S-VIEW-9 velocity->amp transfer curve; eval at start)
 
 namespace reasampler {
 
@@ -199,6 +200,12 @@ struct KeyZone {
     // key plays root pitch); 2.0 = double-rate tracking. Scales the (note-root) semitone offset
     // in the repitch math (keyTrackedRatio); rides BOTH engines via the voice's baseRatio_.
     double keyTrack = 1.0;
+    // S-VIEW-9 velocity->amp transfer curve: maps the note-on velocity (0..127) to the voice's amp
+    // gain, replacing the fixed linear velocity/127. A per-zone performance characteristic (mirror
+    // of keyTrack), carried from PerformanceZone by resolvePerformance and eval'd ONCE in
+    // Voice::start (never per frame). DEFAULT flat y=1 (R10-F1 Option A) — every velocity plays at
+    // unity, a deliberate behavior change from the pre-r10 linear map.
+    vst::VelocityCurve velocityCurve = vst::VelocityCurve::flat();
     std::size_t sampleIndex = 0;  // index into Keymap::samples
 };
 
@@ -370,8 +377,11 @@ public:
     // is default (Gate + Varispeed + no pitch env).
     // `keyTrack` (S-VIEW-6) scales the (note-root) semitone offset feeding the repitch ratio;
     // 1.0 (the default) is standard 12-tone-ET, bit-identical to the pre-S-VIEW-6 baseRatio_.
+    // `velocityCurve` (S-VIEW-9) maps the note-on velocity to the voice's amp gain, evaluated ONCE
+    // here (off the per-frame path); defaults to flat y=1 (R10-F1) — every velocity plays at unity.
     void start(int note, int velocity, const SampleData& sample, int rootNote,
-               double keyTrack = 1.0);
+               double keyTrack = 1.0,
+               const vst::VelocityCurve& velocityCurve = vst::VelocityCurve::flat());
 
     // Gate off — begins the amplitude release. In GATE mode this enters the AHDSR release; in
     // TRIGGER mode it is a NO-OP (Trigger ignores note-off and plays through to its play length).