Renders only while mode === 'point' (armed) and only on a sheet the pick is allowed to reach
(ReferenceModeState.scope). Otherwise it renders nothing and registers no responder, leaving the
primary header selection fully in charge. The producer's gates — scope and allowMulti — are read
here exactly as SheetReferenceLayer reads them for the body: the two surfaces drive one
selection, so a gate honored by only one of them is no gate at all. It is a pointer surface
only (HeaderSelectionLayer with renderBorders={false}) — the reference rectangles are drawn on
the body grid by SheetReferenceLayer; preserveFocus keeps the formula editor focused across a
header drag; replaceActive re-scopes/inserts the active reference instead of the primary's whole-
selection replace.
The header counterpart to SheetReferenceLayer: lets a header click/drag pick a whole row/column reference into the current formula (or chart data range) while POINT mode is armed. A header overlay (mounted per header by
SheetHeaderElement), it reads the app-wide reference-mode bus (useReferenceMode) — not any edit session — and drives the SAME picker IRangeSelection as the body layer, so a header pick and a body draw write back through one model.