feat(i18n): add typed plural and select message variants

[LMLiam]

Outcome

Define and implement typed plural and select message variants after the base registry and generated catalogue are stable.

Design gate

Before adding a dependency or public type, compare these options with real catalogue call sites:

1. a small Kotventure closed model for only the categories required by the accepted use cases;
2. Unicode Common Locale Data Repository (CLDR) plural rules through an established library;
3. International Components for Unicode (ICU) message support.

Show dependency size, supported locales, rule-update ownership, runtime cost, generated API shape, and failure behaviour. Obtain maintainer approval. Do not add ICU or CLDR data speculatively.

Required contract

• The catalogue compiler owns variant schemas and generated placeholder types.
• Every locale must use the same placeholder name and type for a catalogue key.
• Select keys form a closed generated set. Unknown values fail; they do not select the last branch.
• Plural categories come only from the approved rule source. Do not infer categories from English.
• Require an explicit other branch for plural and select variants.
• Validate missing, extra, and unreachable branches at build time where the approved rule model permits it.
• Runtime lookup remains fail-fast for malformed generated or external data.

Acceptance criteria

• The maintainer approves the rule source, dependency, and generated call sites.
• The compiler generates typed plural and select arguments.
• Cross-locale placeholder and branch schemas are checked before generation.
• Required other, unknown select, missing branch, and locale-rule cases have tests.
• Runtime code does not embed an unreviewed English-only rule.
• Documentation states rule-data version and update ownership.

Dependencies

Blocked by #48, #49, and the catalogue-completeness child of #322. Implement diagnostics before runtime variants when useful.