Re-derive the floor-size test fixtures from the constants and move the ceiling to sample_bands.h

src/core/instrument/ui/knob_deck.h

@@ -55,10 +55,10 @@ inline constexpr int kDeckGroupH =
 // include edge to this header.
 inline constexpr int kDeckRowBlockW = 1020;  // the block both categorical rows justify inside
 inline constexpr int kDeckSpanningW = 142;   // the right-anchored spanning deck, outside the block
-// The hard ceiling the FLOOR may not exceed; the window itself still grows freely above it.
-// The gap between it and kEditorMinWidth is the whole width budget for the life of this
-// layout — see instrument-control-surface.md §1.6 before spending any of it.
-inline constexpr int kEditorCeilingWidth = 1280;
+// The hard ceiling the floor may not exceed lives beside the floor itself, in sample_bands.h's
+// kEditorCeilingWidth — a window fact, not a deck one. Today's gap between the two is 90px,
+// the whole width budget for the life of this layout (asserted in test_deck_groups.cpp) — see
+// instrument-control-surface.md §1.6 before spending any of it.
 
 // A two-segment compact toggle (always 2 segments — the Mono/Stereo grammar). id -1 = absent.
 struct DeckToggleDesc {