Update table row cells

A FIELD-SCOPED patch: each entry writes one cell, so two callers editing different cells of one row never clobber each other. Row identity is recomputed over the merged row, so a partial edit cannot give the row a new, wrong identity. null clears a cell. expectedVersion is the row version you read, judged PER CELL: the patch lands unless a cell it writes was changed by another edit after that version, in which case it is a 409 ROW_CELL_CONFLICT whose details name each such cell with its current value ({ theirs, theirVersion } as JSON) and nothing is written. A change to some other cell of the row is not a conflict, and a retried patch that finds its own value already in the cell is not one either. A version newer than the row is a 409 ROW_VERSION_CONFLICT. Omitting expectedVersion overwrites whatever the cells hold.