feat: heading-aware scroll and buffer-change handling

Fix scroll/buffer-switch behavior for heading overlays:

- on-window-change: preserve heading overlay position caches
  (images clear theirs for re-placement, but headings need
  old→new comparison to erase multicell blocks properly)
- on-buffer-change: erase heading multicell blocks when their
  buffer becomes hidden, preventing ghost artifacts if the new
  buffer's content doesn't fully overwrite the heading cells
- on-buffer-change: preserve heading caches for visible buffers
  (same rationale as on-window-change)

The existing two-phase refresh already handles scroll correctly:
phase 1 detects position changes and erases moved headings,
phase 2 re-emits all visible headings.  These fixes ensure the
cache state is correct for that comparison to work.

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