[Feature]: Promote i18n to Stable + message-extraction tooling and documented ICU coverage #141

Pre-flight checklist

• I searched existing issues and did not find a duplicate request.
• I explained the problem or use case, not just the desired API.

Problem or use case
i18n is Beta and grew in 1.14.0 (negotiateLocale, detectLocale, isRTL, formatRelativeTime, formatList, formatDisplayName, segment). First-party i18n is a real differentiator (React/Vue/Svelte mostly lean on third-party libs), but two gaps block both stability and adoption: there is no message-extraction tooling to pull translatable strings out of source, and the extent of ICU / complex-plural coverage is not documented. Teams can't commit to a localization workflow they can't extract for or whose plural support is unspecified.

Proposed solution
Freeze the formatting/locale API for one cycle, add an optional message-extraction tool that scans source for translation calls and emits/merges catalogs, and publish a precise statement of ICU MessageFormat coverage (plurals, selects, nested arguments) with examples — implementing any documented-but-missing cases. Keep extraction build-tool-agnostic and optional to preserve the zero-build path. Exit criteria: frozen API, extraction tool shipped, ICU coverage documented and tested, lazy-loading of catalogs documented.

Possible API or UX shape

import { defineMessages, t } from "@bquery/bquery/i18n";

const messages = defineMessages({
  cart: { items: "{count, plural, one {# item} other {# items}}" }
});

// Optional extraction (any bundler / CLI):
// bquery-i18n extract "src/**/*.ts" --out locales/en.json

Alternatives considered
Documenting that users should manage catalogs by hand — viable for tiny apps but a non-starter at scale and weaker than the mature i18n ecosystems around React/Vue. Shipping extraction as a mandatory build step — violates zero-build; it must be optional.

Relevant area
i18n

Additional context
This is both a stabilization and a competitive-strength ticket: getting extraction + documented ICU coverage turns first-party i18n from "nice to have" into a real reason to choose bQuery.