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_knob_deck.cpp

@@ -156,9 +156,14 @@ static void testHitTest() {
     const DeckGroupLayout& tg = tl.groups[0];
     CHECK(tg.cells.size() == 3);
     for (const DeckCellLayout& c : tg.cells) CHECK(c.id >= 0);
-    const DeckCellLayout& last = tg.cells.back();
-    for (int px = tg.cells[0].cell.x; px < last.cell.right(); ++px) {
-        const DeckHit rowHit = hitTestDeck(tl, px, last.cell.y + 5);
+    // Bound the sweep against the RESERVED run (5 slots, not the 3 present cells) rather than
+    // the cells' own extent — the cells are what's under test, so deriving the bound from them
+    // could never catch a layout that under-covers the run they were reserved out of.
+    const int runStart = tg.box.x + kDeckGroupPadX;
+    const int runEnd = runStart + static_cast<int>(trig[0].cellIds.size()) * kDeckCellW;
+    const int rowY = tg.cells.back().cell.y + 5;
+    for (int px = runStart; px < runEnd; ++px) {
+        const DeckHit rowHit = hitTestDeck(tl, px, rowY);
         CHECK(rowHit.kind == DeckHitKind::Knob && rowHit.id >= 0);
     }
 
@@ -214,6 +219,31 @@ static void testReservedCellWidthGoesToTheCellsPresent() {
           layoutDeck(b, 0, 0, 824).groups[0].rowToggle.seg0);
 }
 
+// The three faces above (240/3, 240/4, 240/1) all divide their run evenly, so none of them
+// actually exercises "residue in symmetric end margins" — a 7-slot reserve with 5 present
+// (336/5, remainder 1) does, and pins the residue split across BOTH ends rather than only
+// the leading one.
+static void testIndivisibleResidueLandsInSymmetricEndMargins() {
+    const DeckGroupDesc g{0, 78, {}, {100, 44}, {}, {20, 21, 22, 23, 24, -1, -1}, {}};
+    std::vector<DeckGroupDesc> gs{g};
+    const DeckLayout dl = layoutDeck(gs, 0, 0, 824);
+    const DeckGroupLayout& lay = dl.groups[0];
+    CHECK(lay.cells.size() == 5);
+
+    const int run = 7 * kDeckCellW;
+    const int present = 5;
+    const int cellW = run / present;               // 67: the same integer division the layout uses
+    const int expectedResidue = run - cellW * present;  // 1: the case the even-dividing faces can't reach
+    CHECK(expectedResidue > 0);
+
+    const int covered = lay.cells.back().cell.right() - lay.cells.front().cell.x;
+    CHECK(run - covered == expectedResidue);
+    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 == expectedResidue / 2);
+    CHECK(trailPad == expectedResidue - leadPad);  // both ends share it, not one absorbing it
+}
+
 // The corner radio widens the caption row, takes the far corner, and pushes the caption
 // toggle left of itself — the three properties the overlay-select switch relies on.
 static void testCaptionRadioGeometryAndHit() {
@@ -307,6 +337,7 @@ int main() {
     testGroupInnerGeometry();
     testHitTest();
     testReservedCellWidthGoesToTheCellsPresent();
+    testIndivisibleResidueLandsInSymmetricEndMargins();
     testCaptionRadioGeometryAndHit();
     testInnerDialHit();
     testCaptionToggle2();