feat(intl): use a plain string translation for every plural form and match case

[dangreen]

Why

Translation data is Record<string, any> at runtime, so a locale file can hold a plain string where the scheme uses plural or match, for example a Japanese translation attendees: '{count}人参加' next to an English { one, other } object. That string reached the key resolvers untouched:

plural('count')(ctx, '{count} going')({ count: 3 })             -> TypeError: Cannot use 'in' operator to search for '3' in {count} going
plural('count', forms({ one, other }))(ctx, '{count} going')(3) -> "u"        (the string was spread into characters)
match('kind', other('b'))(ctx, 'str')({ kind: 'a' })            -> TypeError

What

• match short-circuits a string translation: it becomes the message for every case and every plural form, {param} placeholders included, and the cases(...) / forms(...) resolvers and key resolvers are skipped for it.
• The input types of match and plural accept string, so typed translation data with a string on such a key passes type-checking.
• The API page mentions the rule in the plural and match sections.
• Tests: three for match (bare, with cases, with other), two for plural (bare, with forms).

Wrapping the string as { other: input } was considered and rejected: a forms({ one, other }) resolver declares one with an undefined value, so count: 1 would still resolve to undefined, and match without other('other') never looks at the other key.

Size

The extra branch costs bytes in match, which is part of the All publics and Basic set bundles. Limits are re-pinned on the 0.05 kB step:

bundle	before	after	limit
All publics (Gzip)	1993 B	2009 B	2 kB → 2.05 kB
All publics (Brotli)	1819 B	1831 B	1.85 kB
Basic set (Gzip)	998 B	1013 B	1 kB → 1.05 kB
Basic set (Brotli)	888 B	899 B	900 B

Checks

• oxlint, tsc --noEmit, vitest run (122 tests) and size-limit in packages/intl pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.28%. Comparing base (6b3d44e) to head (4c6cf6f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   83.32%   83.28%   -0.04%     
==========================================
  Files          98       98              
  Lines        2554     2554              
  Branches      549      550       +1     
==========================================
- Hits         2128     2127       -1     
  Misses        314      314              
- Partials      112      113       +1     

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