feat(NitroInput): tabularNums for tabular digits in any font; Android edit text asks for tnum

[ronickg]

Problem

In a font with proportional digits, the keepPlaceholder tail shifted while typing. Reproduced with the example's Open Runde, which is byte-identical to Uno's:

• Typing 1171178 into a 0000 0000 0000 mask moved the tail's end from 172.6 to 158.1 dp on Android and from 173.0 to 159.7 pt on iOS.
• The system font (Roboto / SF) didn't move.

Open Runde ships no tnum at all; the request is open upstream as lauridskern/open-runde#10. Uno's ABC Diatype has tnum, but it's off by default, and Android's edit text never asked for it.

Changes

tabularNums prop

value	what happens	cost
unset (default)	The font's tabular figures, where it has them (system fonts do). This is the old behaviour.	none
true	Every digit as wide as the widest even in a font without tabular figures (Open Runde), centred, as NitroNumber's columns are. Applies to the overlay, the field's own text, the placeholder, the tail and the JS-thread measure.	a little per change, see below
false	The font's proportional figures. A style's fontVariant: ['proportional-nums'] means the same.	none

Natively these are two props: tabularNums (request tnum) and slotDigits (lay digits out in slots).

Android: tnum on the edit text (always on)

The overlay's paints already asked for tnum, but the edit text didn't. That alone fixes the shift for Diatype.

How the slots work

• Android:
  • One ReplacementSpan per digit, and only the changed spans are touched per keystroke. The spans are NoCopySpan, so they never leave with copied text.
  • Letter spacing was tried first and doesn't work: from API 35, Minikin trims it at line edges (letter_spacing_justification).
• iOS:
  • Per-character .kern, plus a first-line indent for the first digit's leading half.
  • When two neighbouring glyphs both carry a non-zero kern, the font's pair kerning comes back (Open Runde kerns "74" by −0.058 em), so that kerning is subtracted.
  • The attributed text is re-set synchronously in the edit handler: skipped during marked text, selection restored.

Verification

Tail end per keystroke with tabularNums, typing 7474 1178 (dp / pt):

field	Android	iOS
Open Runde Regular	181.0 ×8	180.7–181.0
Open Runde Medium / Semibold / Bold	constant	within 0.4
CLABE mask, 16 pt	202.7 ×8	201.7–202.0
reflow	180.6 ×8	181.0 ×8
ABC Diatype Bold, unset (Android only, trial font not committed)	181.7 ×7	–

• Test suites:
  • Harness: iOS 152/152, Android 148/148. New tests check:
    • with tabularNums, an Open Runde mask keeps its width whatever digits are typed (plain and reflow), plus a first-layout case;
    • unset or false, digits keep their own widths.
  • Jest: 190 passed, including the prop → native and measure mapping and fontVariant. Typecheck clean.
• Fast typing (iOS): 9 bursts of 12 digits each, no keys dropped. RN saw drops when setAttributedText: ran async on a focused field ([iOS][Fabric] TextInput drops a keystroke right after the first character typed into an empty field (_textOf:equals: compares attributes of a focused field) react/react-native#58904); ours runs inside our own edit handler.
• Cost per keystroke with tabularNums in Open Runde:
  • Android: span update 0.19 ms median, draw +0.25 ms.
  • iOS: about 0.4 ms. When the text ends in a space (the mask's separator), UIKit's keyboard relayout on the re-set adds about 5–7 ms on the simulator.
  • Unset or false: none.

Known limits

• With tabularNums and a non-zero letterSpacing on Android 15+, the mask's separator space still moves the tail by about 1 dp, because the platform trims that space's spacing.
• Letters stay proportional. Only digits are tabular.

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