i18n: dependency-free message runtime, catalog checks, plugin strings, native strings

- runtime/i18n.ts: ICU-lite messages (placeholders as text, Intl.PluralRules plurals with exact
  forms, selectordinal, select, number/date/time), fallback chains (pt-BR -> pt -> en, ar-x-fm ->
  ar), X and legacy code aliases, RTL by locale, lazy catalogs, live onChange. Apostrophes are
  literal except right before { } # or another apostrophe.
- runtime/i18nCheck.ts: rules for contributed catalogs (shape, keys, syntax, placeholder parity,
  select options, markup/control characters, lengths). An argument English only uses to choose a
  plural form may be dropped; missing plural forms are warnings; meta "allowEmpty" fragments.
- runtime/pluginI18n.ts: api.i18n bound per plugin (register/t/locale/dir/onChange/number/date/
  relative/list); the UI kit's own words follow the app's language. pageLang.ts reports the
  site's <html lang>. shell/messages.ts: shell-model words.
- waybetter-host i18n.rs: NativeStrings (validated plain text, caps, one-pass {name} fill, page
  payload by prefix), system_locales via sys-locale; TabState.lang.
- JSON Schemas for catalogs and translator notes; npm exports waybetter/i18n, /i18n/check,
  /i18n/plugin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/waybetter-host/src/lib.rs

@@ -14,10 +14,12 @@
 //! - [`update`]: silent-update scheduling and the crash handshake.
 //! - [`tabs`]: tab layout/state payloads, link routing and tab-chrome validation.
 //! - [`layout`]: native placement bookkeeping (unchanged bounds skipped, window-resize follow).
+//! - [`i18n`]: the strings native UI and site tabs get in the owner's language (validated data), OS locale.
 //!
 //! With the `tauri` feature, [`host`] adds the Tauri glue that does not depend on app state.
 
 pub mod hotfix;
+pub mod i18n;
 pub mod layout;
 pub mod plugins;
 pub mod policy;