feat: nuke-and-repaint for fold/unfold heading cleanup

Add heading-aware org-cycle handling:

- kitty-gfx--nuke-headings: erases ALL heading multicell blocks
  in the current buffer, clearing their cached positions.  This
  is the "nuke" phase — a clean slate for the terminal.
- kitty-gfx--on-org-cycle: now handles both image and heading
  overlays.  Images get their placements deleted (existing logic),
  headings get their multicell blocks erased (nuke phase).
  The subsequent scheduled refresh is the "repaint" phase:
  kitty-gfx--overlay-screen-pos checks in-folded-region-p, so
  folded headings return nil and won't be re-emitted, while
  visible headings get fresh OSC 66 emission at correct positions.

The nuke-and-repaint strategy avoids incremental erase complexity
(which failed in two prior prototypes) by simply erasing everything
and letting the refresh cycle figure out what's still visible.

0 files changed, 0 insertions, 0 deletions