need to repeatedly resolve a complete neighboring face here, or could it reuse information the display iterator already knows?

The small cache is a proof of concept, not necessarily the right upstream design. It may be better to reuse existing iterator state or provide a narrower way to answer the box-boundary question.

I would especially welcome feedback on:

* Whether there is existing interval information that makes this cache unnecessary.
* Which face-resolution or invalidation cases this approach might miss.
* Whether a box-specific query would be preferable.
* How best to test glyph and border equivalence before proposing a patch.

**Tested environment:** macOS/ARM64, NS backend, Emacs source revision `fc9cf69afa90990776526a7cf9e693893739fc45`, Org 9.8.7, org-modern 1.15. All core modifications were made in a separate experimental build.

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