instrument: fix AHD DecayEnd overlay/grab defect, pin flaky curve test, close comment/doc findings

src/core/instrument/ui/envelope_edit.h

@@ -37,7 +37,9 @@ struct EnvClampBounds {
 // inputs buildEnvelopePolyline drew from). `hit` is false for a point off every draggable node.
 // Nearest node within the radius wins (Chebyshev distance); an exact tie goes to the earlier
 // draw-order node, and since knots are appended last, a coincident endpoint handle wins over a
-// knot rather than the drag silently becoming a curve edit.
+// knot rather than the drag silently becoming a curve edit. An AHD's DecayEnd is excluded
+// entirely when it coincides with HoldEnd AND holdFraction == 1.0 — there it cannot move
+// (resolveNodeDrag's derivative is 0), so it is dropped rather than won by draw order.
 struct NodeHit {
     bool    hit  = false;
     EnvNode node = EnvNode::Origin;  // meaningful only when hit == true