Fix envelope-overlay knot/trace disagreement on odd pixel spans

Generalize curveMidLevel/curveFromMidLevel to curveLevelAt/curveFromLevelAt at
arbitrary phi; knotVtx and its drag inverse now read the phi a knot's truncated
x actually implies, not always 0.5.

tests/test_curve_law.cpp

@@ -14,6 +14,7 @@
 
 #include <cmath>
 #include <cstdio>
+#include <initializer_list>
 
 using namespace reasampler::util;
 
@@ -107,6 +108,39 @@ static void testMidLevelInverseSaturates() {
     CHECK(std::fabs(curveFromMidLevel(0.5) - kCurveNeutral) < 1e-12);
 }
 
+// curveLevelAt/curveFromLevelAt is the general form a knot's own (possibly off-centre) phi
+// needs — curveMidLevel/curveFromMidLevel is the phi = 0.5 case, not a second law.
+static void testMidLevelIsThePhiHalfSpecialCase() {
+    for (double e : {kCurveMin, 0.3, kCurveNeutral, 2.0, kCurveMax}) {
+        CHECK(curveLevelAt(0.5, e) == curveMidLevel(e));
+    }
+    for (double u : {0.0, 0.2, 0.5, 0.8, 1.0}) {
+        CHECK(curveFromLevelAt(0.5, u) == curveFromMidLevel(u));
+    }
+}
+
+// The round trip must hold at an arbitrary phi, not only 0.5 — this is what a knot whose
+// integer x lands off its segment's true midpoint (an odd pixel span) actually exercises.
+static void testLevelAtRoundTripsAtArbitraryPhi() {
+    for (double phi : {0.1, 0.3, 0.42, 0.5, 0.63, 0.9}) {
+        for (int i = 0; i <= 50; ++i) {
+            const double e = kCurveMin + (kCurveMax - kCurveMin) * (i / 50.0);
+            const double level = curveLevelAt(phi, e);
+            CHECK(level > 0.0 && level < 1.0);
+            CHECK(std::fabs(curveFromLevelAt(phi, level) - e) < 1e-9);
+        }
+    }
+}
+
+// Saturation holds at an arbitrary phi too, not only the mid-level special case.
+static void testLevelAtInverseSaturatesAtArbitraryPhi() {
+    for (double phi : {0.2, 0.5, 0.8}) {
+        CHECK(curveFromLevelAt(phi, 0.0) == kCurveMax);
+        CHECK(curveFromLevelAt(phi, 1.0) == kCurveMin);
+        CHECK(curveFromLevelAt(phi, std::nan("")) == kCurveMax);
+    }
+}
+
 // --- The inner dial's travel ---------------------------------------------------
 
 // The knob drag delivers `start - dy/kKnobDragRangePixels`. param_slider owns that constant and
@@ -184,6 +218,9 @@ int main() {
     testClampCurveHoldsTheDomain();
     testMidLevelRoundTripsAgainstTheExponent();
     testMidLevelInverseSaturates();
+    testMidLevelIsThePhiHalfSpecialCase();
+    testLevelAtRoundTripsAtArbitraryPhi();
+    testLevelAtInverseSaturatesAtArbitraryPhi();
     testKnobLawIsExactAtTheNeutralCentre();
     testADialSweptThroughNeutralLandsOnTheIdentity();
     testKnobLawRoundTripsOutsideTheDetent();