feat: emit OSC 66 for heading overlays

Add the OSC 66 emission pipeline (Step 5 of incremental plan):

- kitty-gfx--place-heading: emits \e]66;s=S:n=N:d=D;TEXT\a with
  SGR bold + 24-bit foreground color at cached terminal position
- kitty-gfx--erase-heading: erases multicell blocks by writing
  spaces at the topmost row (overwrite Rules 2-3)
- kitty-gfx--refresh-heading-overlay: phase 1 computes position,
  erases on move/hide, caches new position (no emission)
- kitty-gfx--emit-heading-overlays: phase 2 walks all buffers,
  emits OSC 66 for visible heading overlays after all posn-at-point
  queries complete (prevents redraw-induced multicell destruction)

Two-phase refresh in kitty-gfx--refresh: phase 1 (walk-windows)
does position queries and image placements, phase 2 (after loop)
emits all heading OSC 66 sequences.  Both phases run inside
synchronized output (BSU/ESU) to prevent flicker.

Also adds org-mode integration hooks (heading-modified debounce,
org-mode-hook for auto-apply, heading scan/remove commands) and
self-tests for place-heading and erase-heading in dry-run mode.

0 files changed, 0 insertions, 0 deletions