vst: curve velocity_curve eval with a monotone cubic Hermite (Fritsch-Carlson) spline

True curved response, provably no overshoot outside [0,1]; collinear knots reproduce the exact linear ramp so linear()'s null-response stays bit-exact. Eval-only; v7 wire-format untouched. Adds spline tests.

src/vst/velocity_curve.h

@@ -59,11 +59,13 @@ struct VelocityPoint {
 inline constexpr int kCurveNodeGrabRadius = 6;
 
 // A velocity->amp transfer curve: an X-ORDERED list of control points spanning [0,127], evaluated by
-// LINEAR interpolation between adjacent points (a monotonic polyline: each velocity maps to exactly
-// one amp). Linear-between-knots is deliberate — it makes linear() an EXACT straight line y =
-// velocity/127 (the Option-B / null-response contract) and keeps monotonicity trivial; the "curved"
-// shape a sound wants comes from placing more control points, not from bending one segment. The two
-// endpoints (velocity 0 and 127) are load-bearing: they keep eval total and are never deletable.
+// a MONOTONE cubic Hermite spline (Fritsch–Carlson slope limiting) through the knots — a genuine
+// curved response (Daniel 2026-07-27: "straight lines sound like shit"), not a polyline. Each
+// velocity still maps to exactly one amp: the interpolant is single-valued and provably stays within
+// each segment's amp range, so the curve never overshoots below 0 or above 1. For COLLINEAR knots the
+// Fritsch–Carlson tangents reduce to the secant slope, so the spline IS the straight line — that
+// keeps linear() an EXACT y = velocity/127 (the Option-B / null-response contract). The two endpoints
+// (velocity 0 and 127) are load-bearing: they keep eval total and are never deletable.
 class VelocityCurve {
 public:
     // R10-F1 default (Option A): flat y=1 — endpoints (0,1) and (127,1); every velocity -> unity.
@@ -86,8 +88,11 @@ public:
 
     // Evaluate the curve at `velocity` -> amp in [0,1]. Velocity is box-clamped to [0,127] first,
     // so an out-of-range note (shouldn't occur) reads the nearest endpoint. Between two adjacent
-    // points the amp interpolates LINEARLY across the normalized X position — monotonic in X. A
-    // degenerate curve (0 or 1 point, shouldn't occur post-construction) returns kAmpMax (flat).
+    // points the amp follows a MONOTONE cubic Hermite spline (Fritsch–Carlson slope limiting) — a
+    // true curve that provably stays within the two knots' amp range (no overshoot below 0 / above
+    // 1) and reduces to the exact straight line for collinear knots. Single-valued / monotonic in X.
+    // Degenerate cases (shouldn't occur post-construction): an EMPTY curve returns kAmpMax (flat
+    // unity); a ONE-point curve returns that point's amp.
     double eval(double velocity) const;
 
     // --- Editing (for the S-VIEW-10 editor UI) --------------------------------------------------