Four fixes to how `digits` tate-chu-yoko composites lay out in vertical
text:
- A lone digit combines too, as CSS `digits <n>` combines runs of n or
fewer, so it stands upright instead of turning sideways. The renderer
(`split_digit_runs`) and the export path (`digit-combine-segments`)
share the rule.
- Composites centre on the OS/2 typographic metrics, like upright glyphs.
The hhea ascent of CJK faces exceeds the em, which set digits about
0.07em below the kanji beside them.
- Letter-spacing stays outside the composite, as in CSS. Counting the
trailing spacing in its width moved the digits left by half of it.
- The regional Chinese profiles no longer add the Han/Western gap around
a composite: it sets in one ideograph's cell, as JLREQ already does.
AI-assisted-by: claude-opus-5-5
b053ac8751