Close gap #9 (cells hosting widgets) the spreadsheet-native way. The
datagrid reference hosts a live CheckBox widget per visible cell and
recycles instances from a per-template pool; our grid instead draws the
control from batched quads, so there is no per-cell widget object to
instantiate or recycle — the render cache and reused draw buffers already
fill the role the pool does.
- A plain value cell holding TRUE/FALSE now renders as a checkbox: a
centred square box with a checkmark when TRUE, and its TRUE/FALSE label
to the right. A single click toggles the value (through the normal
set_cell path, so it is undoable and recalculates dependents), selects
the cell, and does not open the editor — a checkbox is a button, not a
text surface. Formula cells are never checkboxes, so a click can't
clobber a formula.
- The predicate (is_checkbox), the value flip (toggled) and the box
geometry (checkbox_layout) live in a new measured checkbox module with
unit tests; grid.rs stays thin glue (draw the box/tick/label, reposition
the label, and the click handler's toggle).
UI controllers: 96 tests (+6). Coverage: ui-controllers 99.20% (floor 96),
checkbox.rs at 100%. No engine changes.
