Single-character tate-chu-yoko (unrotated letters and numbers in vertical text)

[1ec5]

Following up from #3506 and #3438 (comment), when a label is vertically oriented with a vertical writing mode, any one- or two-digit numbers (and one- or two-letter words) would ideally be left upright, their digits placed side-by-side. The same should happen for one- or two-letter runs of Latin text. This effect is called tate-chu-yoko in Japanese:

Currently, we replace a punctuation character with its fullwidth analogue if both adjacent characters are compatible with vertical writing. This heuristic is conservative enough that we could implement single-character tate-chu-yoko simply by mapping all the ASCII numbers and letters to their fullwidth analogues in lookup (or leftAndRightBinding once #3587 is implemented), or by adding 0xfee0 to the character code of any ASCII number or letter.

/cc @lucaswoj @nickidlugash @friedbunny

[1ec5]

Double- or triple-character tate-chu-yoko would require the special logic to avoid rotating ASCII characters, position them within the correct half-character box, and use specialized characters like ㏾. While impressive, I think anything beyond single-character tate-chu-yoko is unnecessary for the time being.