instrument: fix AHD node-tracking/tie-break/live-latch defects and close staged-envelope-curve test gaps

tests/test_envelope_edit.cpp

@@ -193,9 +193,20 @@ static void testAhdStageTimesTrackTheWallClockScale() {
     const StageEnvelope attack =
         resolveNodeDrag(e, EnvNode::AttackEnd, overlayOf(a), kTotal, bounds(), 100, 0);
     CHECK(std::fabs(attack.attackSeconds - (e.attackSeconds + 100 * secPerPx)) < 1e-9);
+
+    // DecayEnd's underlying param (decaySeconds) does NOT move 1:1 with the cursor: the drawn
+    // endpoint is t0 + total, and Hold eats a holdFraction share of whatever decay gives up
+    // (d(total)/d(decay) = 1 - holdFraction), so decaySeconds itself has to move faster than
+    // the cursor to make the DRAWN node track it. Assert on the RENDERED position, not the
+    // raw param — that is the property a drag actually has to deliver, and asserting the old
+    // 1:1 param delta here is exactly what let the node-tracking defect through undetected.
+    EnvVertex before;
+    CHECK(findNode(buildEnvelopePolyline(e, overlayOf(a), kTotal), EnvNode::DecayEnd, before));
     const StageEnvelope decay =
         resolveNodeDrag(e, EnvNode::DecayEnd, overlayOf(a), kTotal, bounds(), 100, 0);
-    CHECK(std::fabs(decay.decaySeconds - (e.decaySeconds + 100 * secPerPx)) < 1e-9);
+    EnvVertex after;
+    CHECK(findNode(buildEnvelopePolyline(decay, overlayOf(a), kTotal), EnvNode::DecayEnd, after));
+    CHECK(std::abs((after.x - before.x) - 100) <= 1);  // 1:1 with the cursor, to rounding
 }
 
 // Hold is a fraction of what attack and decay left, so the node's pixel motion converts through
@@ -270,6 +281,19 @@ static void testKnotOnALevelSegmentIsANoOp() {
     CHECK(out.decayCurve == 2.5);
 }
 
+// A NEAR-level segment (sustain 0.99) is not caught by the exact-equality guard above, but its
+// tiny divisor turns a one-pixel drag into a saturating swing of the exponent — the drag must
+// still be a no-op rather than slam to a domain endpoint.
+static void testKnotOnANearLevelSegmentIsANoOp() {
+    const Rect a = wideArea();
+    StageEnvelope e = ahdsrEnv();
+    e.sustainLevel = 0.99;
+    e.decayCurve = 2.5;
+    const StageEnvelope out =
+        resolveNodeDrag(e, EnvNode::DecayCurve, overlayOf(a), kTotal, bounds(), 0, -1);
+    CHECK(out.decayCurve == 2.5);
+}
+
 // --- degenerate ----------------------------------------------------------------
 
 static void testDegenerateInputsAreNoOps() {
@@ -299,6 +323,7 @@ int main() {
     testKnotDragMovesTheExponentWithinItsDomain();
     testKnotAndModelCannotDiverge();
     testKnotOnALevelSegmentIsANoOp();
+    testKnotOnANearLevelSegmentIsANoOp();
 
     testDegenerateInputsAreNoOps();