fix(renderer): only take the statusline off a window's bottom once

[yal212]

Fixes #383.

With laststatus=2, a normal window never showed an image in its last row. An image whose top was on that row was sent with a crop height of 0, which the kitty protocol reads as the whole image, so it was drawn over the statusline and command line. laststatus=0 and 3 were fine.

Cause

Two terms applied only when laststatus == 2:

• get_windows() took a row off rect.bottom for the statusline, but nvim_win_get_height() already excludes it. The renderer's bounds.bottom - 1, which turns the exclusive bottom into the last row, then counted that row a second time.
• is_below added the row back (+ laststatus_offset), so an image one row below the bounds still counted as visible and was cropped to (bounds.bottom - y + 1) = 0 rows.

Change

• lua/image/utils/window.lua: rect.bottom = rect_top + content_height for every laststatus. This also makes the float-overlap masks, which already treat rect.bottom as exclusive, correct for the last row.
• lua/image/renderer.lua: is_below = absolute_y > bounds.bottom, with the now-unused laststatus_offset removed. The renderer's - 1 stays.

Fixing only window.lua would bring back the statusline overdraw, because is_below would still let an image that starts on the statusline row through. One of the new tests covers that.

Tests

• tests/utils/window_spec.lua: a normal window's rect.bottom is top + height for laststatus 0, 2 and 3.
• tests/renderer/bottom_bounds_spec.lua: in a real window, a 3-row image whose top is on the window's third-to-last, second-to-last and last row is shown with 3, 2 and 1 rows, and one that starts below the window is not rendered, for laststatus 0, 2 and 3.

Without the fix, the laststatus=2 cases fail (2 rows instead of 3, and rect.bottom 19 instead of 20). With it, the full suite passes (103/103, busted + nlua on Lua 5.1, as in CI).

Verified live

This is the repro from #383, kitty backend, Ghostty inside tmux. display_height is in rows at 32 px cells:

laststatus	master	this PR
0	3, 2, 1	3, 2, 1
2	2, 1, 0	3, 2, 1
3	3, 2, 1	3, 2, 1

Both laststatus terms came from #180 (for #179, an image over the statusline with laststatus=2). The renderer's - 1 already keeps images off the statusline, and the last-row case above shows it: the image there is drawn 1 row tall, not over the statusline.