fix: preserve tui headings while scrolling

[rrbe]

Summary

• assign each heading an explicit Kitty placement id
• move headings by replacing the existing placement instead of deleting it first
• delete off-screen placements without evicting cached image data in iTerm2

Verification

• make check
• reproduced with installed termdown 0.5.1 on iTerm2 3.6.10
• verified the patched debug build keeps headings visible after j/k scrolling