fix: close round-2 review findings — Critical silent-note bug plus majors/minors

Fixes the 2-point spline-EG early-free bug causing silent fade-ins, the pitch/filter enable-toggle Trigger-forcing hole, the contour-node/marker pixel shadow, missing deck-residue test coverage, and stale comments in knob_deck and spline_edit.

tests/test_deck_groups.cpp

@@ -565,6 +565,31 @@ static void testNoFaceLeavesSlackWhereItsDroppedControlsWere() {
     }
 }
 
+// Every shipped face's reserve divides its present-cell count evenly (see
+// testNoFaceLeavesSlackWhereItsDroppedControlsWere), so none of them exercises the
+// "residue lands in symmetric end margins" rule knob_deck.cpp documents — only that the
+// leftover is small, not where it goes. A synthetic 7-slot reserve with 5 present (336/5,
+// remainder 1) forces a real residue and pins it split across BOTH ends.
+static void testASyntheticIndivisibleReserveSplitsItsResidueAcrossBothEnds() {
+    const DeckGroupDesc g{0, 78, {}, {100, 44}, {}, {20, 21, 22, 23, 24, -1, -1}, {}};
+    const std::vector<DeckGroupDesc> gs{g};
+    const DeckLayout dl = layoutDeck(gs, 0, 0, kAvailAtMinWidth);
+    const DeckGroupLayout& lay = dl.groups[0];
+    CHECK(lay.cells.size() == 5);
+
+    const int run = 7 * kDeckCellW;
+    const int cellW = run / 5;                // the same integer division layoutGroup uses
+    const int residue = run - cellW * 5;       // 1: nonzero, unlike every shipped face's reserve
+    CHECK(residue > 0);
+
+    const int covered = lay.cells.back().cell.right() - lay.cells.front().cell.x;
+    CHECK(run - covered == residue);
+    const int leadPad = lay.cells.front().cell.x - (lay.box.x + kDeckGroupPadX);
+    const int trailPad = (lay.box.right() - kDeckGroupPadX) - lay.cells.back().cell.right();
+    CHECK(leadPad == residue / 2);
+    CHECK(trailPad == residue - leadPad);      // both ends share it, not one cell absorbing it
+}
+
 // Gate is the common face and it already packs correctly: pin its group widths and row
 // assignment at the floor so a later edit anywhere in the deck cannot reflow it silently.
 // (Measured from the shipped descriptors, not copied out of a failing run.)
@@ -666,6 +691,7 @@ int main() {
     testWrappedDeckHeightAtTheEditorFloorWidth();
     testDeckFitsInsideTheEnforcedMinimumWindow();
     testNoFaceLeavesSlackWhereItsDroppedControlsWere();
+    testASyntheticIndivisibleReserveSplitsItsResidueAcrossBothEnds();
     testGateModeWidthsAndRowAssignmentAreUnchanged();
     testGateSplineGateRoundTripsToTheSameLayout();
     testHitTestResolvesTheNewFilterControls();