Problem: a font set via nvim_set_hl is lost or corrupted with font-only groups,
attribute combining, and update=true, and is dropped on any attr-table rebuild.
nvim__inspect_cell also frees the font name while a returned dict still borrows it.
Solution: register font-only groups, carry font through hl_combine_attr, inherit
it on update, and persist sg_font so a rebuild can restore it. Keep interned font
names across a rebuild instead of clearing them. Add the missing font field to
get_hl_info.
Co-authored-by: Ryan Patterson <cgamesplay@cgamesplay.com>
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