Deleting the predecessor of a shared-row item in a multi-line inline
array or table left the survivor with its one-space inline separator
(e.g. ` 3,`) instead of the canonical row indent, because the
carried-boundary shift had no downstream newline to drop.

A shared-row follower has no row break of its own. Once its new
predecessor terminates its own row, the follower becomes a row leader,
so its inline separator must become the row indent. Handle this in the
shared `_shift_carried_boundary` primitive, so inline arrays, inline
tables, deletions and reorders all benefit. Extract `_value_indent`
(shared with `_canonical_separator`) to sample the indent.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>