instrument: the filter's velocity depth knob returns and multiplies the bipolar curve; the pre-v12 lift is a pure domain re-tag

src/core/instrument/engine/play_params.h

@@ -110,21 +110,23 @@ 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 modulation depths below land in that same normalized cutoff domain and sum before a
-// single clamp; all are zero/neutral by default.
+// 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.
 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], scales velocityCurve's output
     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
-    // 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).
+    // Velocity -> cutoff, in the normalized cutoff domain. The contribution is
+    // velAmount * velocityCurve.eval(velocity): the BIPOLAR curve carries the shape (and its
+    // own sign), the depth knob scales it, and BOTH apply. The curve is flat at 0 by default,
+    // so no depth setting produces velocity modulation until a curve is drawn.
     VelocityCurve velocityCurve = VelocityCurve::zero();
 };