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.

src/core/instrument/ui/knob_deck.cpp

@@ -180,11 +180,12 @@ DeckHit hitTestDeck(const DeckLayout& layout, int x, int y) {
                 return {DeckHitKind::RowToggle, g.rowToggle.id, 1};
         }
         for (const DeckCellLayout& c : g.cells) {
-            if (c.id >= 0 && contains(c.cell, x, y)) {
+            // Every entry here already has a real id — a reserve yields no DeckCellLayout at all.
+            if (contains(c.cell, x, y)) {
                 return {DeckHitKind::Knob, c.id, -1, contains(c.inner, x, y)};
             }
         }
-        return {};  // inside the box but on fence/padding/blank — a miss (groups never overlap)
+        return {};  // inside the box but on fence/padding — a miss (groups never overlap)
     }
     return {};
 }