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_curve_popup.cpp

@@ -6,10 +6,14 @@
 
 #include "../src/core/instrument/ui/curve_popup.h"
 
+#include "../src/core/instrument/engine/velocity_curve.h"
+
 #include <cstdio>
 
 using namespace reasampler;
 using namespace reasampler::instrument::ui;
+using reasampler::instrument::engine::CurveDomain;
+using reasampler::instrument::engine::VelocityCurve;
 
 static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
@@ -80,7 +84,40 @@ static void testOutsideSheetDismissTest() {
     CHECK(!popupOutsideSheet(pl, pl.sheet.right() - 1, pl.sheet.bottom() - 1));
 }
 
+// The sheet hosts all three curves, and its box is domain-agnostic: the SAME curveBox drives a
+// unipolar and a bipolar editor, and only the curve's own y map differs. Asserted here, against
+// the popup's real geometry, because that is what makes one popup code path legitimate.
+static void testTheSameCurveBoxHostsBothDomains() {
+    const CurvePopupLayout pl = computeCurvePopup(840, 620);
+    // The shell insets this rect before mapping; the inset is uniform, so any box inside the
+    // curveBox exercises the same relationship. Use the rect itself.
+    const VelocityCurve::Box box{pl.curveBox.x, pl.curveBox.y, pl.curveBox.width,
+                                 pl.curveBox.height};
+    CHECK(box.width > 1 && box.height > 1);
+
+    const VelocityCurve amp = VelocityCurve::flat();
+    const VelocityCurve mod = VelocityCurve::zero();
+    const int top = box.top;
+    const int bottom = box.top + box.height - 1;
+
+    // Both domains put their MAX on the top row and their MIN on the bottom row...
+    CHECK(amp.pixelFromPoint(box, {0.0, 1.0}).y == top);
+    CHECK(amp.pixelFromPoint(box, {0.0, 0.0}).y == bottom);
+    CHECK(mod.pixelFromPoint(box, {0.0, 1.0}).y == top);
+    CHECK(mod.pixelFromPoint(box, {0.0, -1.0}).y == bottom);
+    // ...so value 0 is the FLOOR for the amp curve and the MIDLINE for a modulation curve.
+    CHECK(mod.pixelFromPoint(box, {0.0, 0.0}).y == (top + bottom) / 2);
+
+    // And a click at the vertical centre adds a point at 0 in the bipolar editor, at 0.5 in
+    // the unipolar one — one hit-test path, two correct answers.
+    const int midY = (top + bottom) / 2;
+    CHECK(mod.pointFromPixel(box, box.left, midY).value == 0.0);
+    CHECK(amp.pointFromPixel(box, box.left, midY).value > 0.49);
+    CHECK(amp.pointFromPixel(box, box.left, midY).value < 0.51);
+}
+
 int main() {
+    testTheSameCurveBoxHostsBothDomains();
     testDefaultWindowMidClamp();
     testMinClamp();
     testMaxClamp();