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.

tests/test_sampler_core.cpp

@@ -1766,7 +1766,8 @@ static SampleData twoLevelSample() {
     km.velocityCurve = VelocityCurve::fromPoints({{0.0, 0.0},
                                                   {static_cast<double>(kVelLow), 0.25},
                                                   {static_cast<double>(kVelHigh), 0.75},
-                                                  {127.0, 1.0}});
+                                                  {127.0, 1.0}},
+                                                 instrument::engine::CurveDomain::Unipolar);
     return km;
 }
 
@@ -1966,6 +1967,65 @@ static SampleData rampSample(std::size_t frames, int rootNote) {
     return s;
 }
 
+// --- velocity -> pitch ---------------------------------------------------------
+//
+// A ramp sample reads its own position, so the value on frame N IS the accumulated read rate:
+// the transpose is directly observable rather than inferred from a spectrum.
+
+static void testVelocityPitchIsExactlyOffByDefault() {
+    // The modulation VALUE at every velocity, not a rendered approximation of it: the default
+    // bipolar curve must yield the identity ratio exactly, so nothing detunes by a hair.
+    const PlayParams def;
+    for (int v = 0; v <= 127; ++v) {
+        CHECK(velocityPitchRatio(def.pitchVelocityCurve, v) == 1.0);
+    }
+    // And at the render: two strikes of very different velocity read the ramp identically.
+    SampleData s = rampSample(4096, 60);
+    Voice soft;
+    Voice hard;
+    soft.start(60, 1, s);
+    hard.start(60, 127, s);
+    for (int i = 0; i < 64; ++i) CHECK(soft.renderFrame() == hard.renderFrame());
+}
+
+static void testDrawnVelocityPitchCurveTransposesBothWays() {
+    using instrument::engine::CurveDomain;
+    SampleData s = rampSample(4096, 60);
+    const double full = std::pow(2.0, kVelocityPitchRangeSemitones / 12.0);
+
+    // A curve pinned at +1 across the domain: every velocity transposes UP by the full scale.
+    s.play.pitchVelocityCurve =
+        VelocityCurve::fromPoints({{0.0, 1.0}, {127.0, 1.0}}, CurveDomain::Bipolar);
+    Voice up;
+    up.start(60, 100, s);
+    CHECK(approx(static_cast<double>(up.renderFrame()), 0.0, 1e-9));
+    CHECK(approx(static_cast<double>(up.renderFrame()), full, 1e-4));
+
+    // Pinned at -1: DOWN by the same scale — the half of the domain the old unipolar curve
+    // could not express at all.
+    s.play.pitchVelocityCurve =
+        VelocityCurve::fromPoints({{0.0, -1.0}, {127.0, -1.0}}, CurveDomain::Bipolar);
+    Voice down;
+    down.start(60, 100, s);
+    down.renderFrame();
+    CHECK(approx(static_cast<double>(down.renderFrame()), 1.0 / full, 1e-4));
+
+    // And it follows the curve: a rising ramp gives a soft hit less transpose than a hard one.
+    s.play.pitchVelocityCurve =
+        VelocityCurve::fromPoints({{0.0, 0.0}, {127.0, 1.0}}, CurveDomain::Bipolar);
+    Voice q;
+    Voice f;
+    q.start(60, 20, s);
+    f.start(60, 120, s);
+    q.renderFrame();
+    f.renderFrame();
+    const double quiet = static_cast<double>(q.renderFrame());
+    const double loud = static_cast<double>(f.renderFrame());
+    CHECK(quiet > 1.0);
+    CHECK(loud > quiet);
+    CHECK(loud < full);  // velocity 120 is short of the +1 endpoint
+}
+
 // MAJOR-1 regression: MONO+LEGATO with a TRIGGER zone RE-ATTACKS after the last key is up.
 // Trigger ignores note-off (Voice::release() is a no-op, so releasing_ never latches), so a
 // legato guard keyed on `active && !releasing` saw a ringing one-shot as "still held" and
@@ -2801,6 +2861,8 @@ int main() {
     testRepitchObservedPeriod();
     testKeyTrackVarispeedObservedPeriod();
     testKeyTrackPreserveShiftCollapsesAtZero();
+    testVelocityPitchIsExactlyOffByDefault();
+    testDrawnVelocityPitchCurveTransposesBothWays();
     testAdsrShape();
     testAdsrReleaseBeforeSustain();
     testAdsrZeroAttackDecay();