sl0p: optionally render the stacked-pane picker as a grid

`stack_list_layout "grid"` packs picker entries several per row, which turns
stack_list_max_rows from a count into a space budget: eight panes in a wide
terminal now fit on one row instead of spending three and hiding five.

The entry renderer already drew each entry as a single chunk at its own x/y,
sized by its geom and centring its own label, so a grid needed no rendering
changes at all -- only the loop assigning every entry the same x and the full
width. Column count is derived from the widest label rather than configured, so
it adapts to the titles; one column is the floor and reproduces the previous
full-width rows byte for byte.

The row -> member mapping now has a column axis, and it had already caused one
wrong-pane bug when two consumers disagreed. So the layout is computed once,
by one function, into explicit cells; the renderer, the reserved-row budget and
the mouse hit-test all read those same cells rather than each doing their own
arithmetic. Windowing is by row, so a grid scrolls a whole row at a time and
degrades to the old per-member window when there is one column.

Cells are separated by a two-column gap: without it the focused entry's "> "
marker sits flush against the previous cell and reads as belonging to it.

0 files changed, 0 insertions, 0 deletions