A phase 0 defect that could only surface once a real catalog used a plural. The type
walked into { one, other } and contributed 'foo.one'/'foo.other' to the key union, so
t('foo', { count }) — the whole point of D7 — did not compile, while t('foo.one') did.
Found by the type checker on the first genuine plural in phase 2 (the rescan toast).
Mirrors isPluralForms at runtime: every key a CLDR category, every value a string.
Pinned with type-level @ts-expect-error assertions, since nothing else would notice.