instrument: a deck group's reserved cell width goes to the cells present

A Trigger face dropping Sustain and Release now gets wider cells instead of
144 px of dead slots. Group widths, row packing and Gate are untouched.

src/core/instrument/ui/knob_deck.h

@@ -5,9 +5,9 @@
 //
 // The deck is a horizontal run of fenced groups, left->right, each a bordered box with a
 // caption row (caption left, the group's compact mode toggle right-anchored) over a knob
-// row of fixed cells (knob centered, label band beneath). A group may also place one
+// row of equal-width cells (knob centered, label band beneath). A group may also place one
 // two-segment toggle in the knob row after its cells. Groups that must keep stable
-// geometry across a mode flip reserve blank cells (id -1) so a mode flip never reflows
+// geometry across a mode flip reserve cell width (id -1) so a mode flip never reflows
 // neighbouring groups.
 //
 // Wrap is deterministic: groups place left-to-right with kDeckGroupGap between; a group
@@ -57,7 +57,8 @@ struct DeckRadioDesc {
 };
 
 // One fenced group, in deck order. `cellIds` are the knob cells left-to-right; an id of -1
-// is a reserved blank cell (geometry held, never hit). `captionWidth` is the px the shell
+// reserves one cell's WIDTH without a cell, and the cells present divide the whole run —
+// see this module's CLAUDE.md bullet for what that buys. `captionWidth` is the px the shell
 // reserves for the caption text (this module does not measure text).
 struct DeckGroupDesc {
     int id = 0;                    // shell group id (opaque here)
@@ -87,7 +88,7 @@ struct DeckRadioLayout {
 
 struct DeckCellLayout {
     int id = -1;
-    Rect cell;   // the full 48x58 cell
+    Rect cell;   // the whole cell; width is the group's reserved run divided by its cell count
     Rect knob;   // the centered kDeckKnobSize square (the knob circle inscribes it)
     Rect inner;  // the concentric kDeckInnerDialSize square inside `knob`
     Rect label;  // the 12px label band beneath the knob
@@ -141,7 +142,7 @@ struct DeckHit {
 // The deck element a point lands on: a knob cell (the whole cell, not just the knob
 // circle — the shell anchors the vertical drag wherever the grab lands, with `inner` marking
 // a grab on the concentric inner dial), a caption-toggle segment, a row-toggle segment, or
-// the caption-row corner radio. Blank cells (id -1) and everything else miss.
+// the caption-row corner radio. Everything else — fence, padding, outside — misses.
 DeckHit hitTestDeck(const DeckLayout& layout, int x, int y);
 
 }  // namespace reasampler::instrument::ui