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_envelope_edit.cpp

@@ -373,6 +373,40 @@ static void testKnotOnANearLevelSegmentIsANoOp() {
     CHECK(out.decayCurve == 2.5);
 }
 
+// The knot drag must read the SAME phi the draw used even off the segment midpoint (an odd
+// pixel span), not the fixed phi = 0.5 wideArea()'s AttackCurve span happens to land on above.
+// Checked two ways: a zero-delta grab reproduces the stored exponent, and a real one-pixel drag
+// moves the knot's own drawn y by the same one pixel every other node axis tracks 1:1.
+static void testKnotDragTracksTheDrawOnAnOddPixelSpan() {
+    bool found = false;
+    for (int width = 24; width <= 260 && !found; ++width) {
+        const Rect a = Rect::ltrb(0, 0, width, 100);
+        StageEnvelope e = ahdsrEnv();
+        e.attackCurve = 3.0;
+        EnvVertex origin, attackEnd, knot;
+        const std::vector<EnvVertex> poly = buildEnvelopePolyline(e, overlayOf(a), kTotal);
+        if (!findNode(poly, EnvNode::Origin, origin)) continue;
+        if (!findNode(poly, EnvNode::AttackEnd, attackEnd)) continue;
+        if (!findNode(poly, EnvNode::AttackCurve, knot)) continue;
+        const int span = attackEnd.x - origin.x;
+        if (span <= 0 || span % 2 == 0) continue;
+        found = true;
+
+        const StageEnvelope same =
+            resolveNodeDrag(e, EnvNode::AttackCurve, overlayOf(a), kTotal, bounds(), 0, 0);
+        CHECK(std::fabs(same.attackCurve - e.attackCurve) < 1e-9);
+
+        const StageEnvelope dragged =
+            resolveNodeDrag(e, EnvNode::AttackCurve, overlayOf(a), kTotal, bounds(), 0, 1);
+        EnvVertex knotAfter;
+        CHECK(findNode(buildEnvelopePolyline(dragged, overlayOf(a), kTotal), EnvNode::AttackCurve,
+                       knotAfter));
+        CHECK(knotAfter.x == knot.x);  // a curve drag never moves the knot's x
+        CHECK(std::abs(knotAfter.y - (knot.y + 1)) <= 1);
+    }
+    CHECK(found);  // the sweep must actually land on an odd span
+}
+
 // --- the interaction law on the overlay ----------------------------------------
 
 // Ctrl scales the PIXEL delta, so it composes with every axis — the tapered schematic, the 1:1
@@ -476,6 +510,7 @@ int main() {
     testKnotAndModelCannotDiverge();
     testKnotOnALevelSegmentIsANoOp();
     testKnotOnANearLevelSegmentIsANoOp();
+    testKnotDragTracksTheDrawOnAnOddPixelSpan();
 
     testDegenerateInputsAreNoOps();