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/deck_groups.cpp

@@ -88,8 +88,9 @@ std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode) {
         amp.captionToggle2 = {id(DeckParam::kAmpEnvMode), kEnvModeSegW};
         if (trigger) {
             // The play span first, then the AHD that shapes it, time-ordered left-to-right so
-            // the row reads like the drawn envelope. One blank keeps the group's width — and
-            // therefore its neighbours' placement — identical across a mode flip.
+            // the row reads like the drawn envelope. One reserve (-1) keeps the group's width —
+            // and therefore its neighbours' placement — identical across a mode flip; its
+            // pixels go to the four cells that remain (knob_deck.h).
             amp.cellIds = {id(DeckParam::kTrigLength), id(DeckParam::kTrigAttack),
                            id(DeckParam::kTrigHold), id(DeckParam::kTrigDecay), -1};
         } else {