fix: close review findings on the velocity-curve deck and bipolar curves

Cancels the curve-node drag whenever the popup closes so Esc mid-drag can't alias the amp curve; generalizes CurveTarget routing to one switch; fixes stale/overstated comments; clamps a pre-v12 depth fold; adds deck-inertness and filter-fold test coverage.

tests/test_deck_groups.cpp

@@ -459,11 +459,29 @@ static void testOverlayIsInertExactlyWhenItsGroupToggleIsOff() {
     CHECK(!overlayEnvInert(OverlayEnv::kNone, false, false));
 }
 
+// A deck knob goes inert exactly with its group's own enable toggle — including the filter's
+// VELOCITY cell, which sits in the VELOCITY group visually but is a filter parameter and must
+// go inert with the rest of the filter (the reachable-through-the-deck route mouseDownDeck
+// checks before ever routing a curve-cell click to the popup).
+static void testDeckKnobIsInertExactlyWithItsGroupsEnableToggle() {
+    CHECK(deckKnobInert(DeckParam::kFilterVelCurve, /*pitchEnv=*/true, /*filter=*/false));
+    CHECK(!deckKnobInert(DeckParam::kFilterVelCurve, true, true));
+    CHECK(deckKnobInert(DeckParam::kFilterCutoff, true, false));
+    CHECK(!deckKnobInert(DeckParam::kFilterCutoff, true, true));
+    CHECK(deckKnobInert(DeckParam::kPitchEnvDepth, /*pitchEnv=*/false, true));
+    CHECK(!deckKnobInert(DeckParam::kPitchEnvDepth, true, true));
+    // The amp's own velocity cell and every ordinary control are never inert here — inertness
+    // is a filter/pitch-env-group-only concept.
+    CHECK(!deckKnobInert(DeckParam::kAmpVelCurve, false, false));
+    CHECK(!deckKnobInert(DeckParam::kAttack, false, false));
+}
+
 int main() {
     testOverlaySelectionIsExclusiveAcrossTheThreeEnvelopeDecks();
     testClickingTheActiveOverlayRadioClearsToNone();
     testANonRadioIdLeavesTheOverlaySelectionAlone();
     testOverlayIsInertExactlyWhenItsGroupToggleIsOff();
+    testDeckKnobIsInertExactlyWithItsGroupsEnableToggle();
     testEveryDeckControlIsClassifiedLiveOrReloading();
     testOnlyALiveControlsDragTakesTheLiveTier();
     testDeckReadsPitchThenFilterThenAmpLeftToRight();