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/map/sample_map.h

@@ -183,11 +183,10 @@ struct FilterSeconds {
     bool enabled = false;
     engine::filter::FilterSettings settings;
     double modAmount = 0.0;
-    double velAmount = 0.0;
     double keyTrack  = 0.0;
     AdsrSeconds env{0.0, 0.0, 0.0, 1.0, 0.0};  // Gate
     AhdSeconds  trigEnv;                       // Trigger
-    VelocityCurve velocityCurve = VelocityCurve::linear();
+    VelocityCurve velocityCurve = VelocityCurve::zero();
 };
 
 // The stored play bundle: wall-clock times in SECONDS, source-timeline quantities in
@@ -200,6 +199,7 @@ struct PlaySeconds {
     AhdSeconds      trigAhd;                              // Trigger amp: AHD (seconds + fraction)
     PitchEngine     pitchEngine = kDefaultPitchEngine;    // product default: Preserve
     PitchEnvSeconds pitchEnv;                             // AHD pitch modulation, off by default
+    VelocityCurve   pitchVelocityCurve = VelocityCurve::zero();  // velocity -> pitch, off by default
     FilterSeconds   filter;                               // per-voice filter, off by default
 };