CellHandle

A CellHandle — the logical, lightweight reference to a single cell of a DataGrid, valid whether or not that cell is currently mounted.

Handles are computed on demand and never stored: the cell-walking APIs (at/slice/cells/queryCells) build them from a (row, column) pair when called, then discard them. This keeps steady-state memory flat with dataset size (no per-cell objects retained) — see the plan's "one source of truth, two projections" model.

It is a thin bundle over the two controllers: rowIndex/cellIndex/record/ value derive from row/column, and element resolves to the live cell iff the row is mounted (it never materializes one). Implemented as a factory rather than a class — there's no state beyond the two references.