[mvilacad]

Closes #325

Design

• The catalog is split per group: i18n/groups/<name>.ts, each defineGroup("name", { en, "pt-BR" }) (50 groups). pt-BR stays typed against en (missing key = type error).
• Components import only the groups they read and call useMessages(group, …) instead of useT(); t().group.key call sites are unchanged. Built-in locales travel inside each group, so SSR stays synchronous and a component pays for its own en + pt-BR strings only.
• catalog.ts (rune-free) holds defineGroup, getGroup(group, locale, override?) and the registerLocale registry; resolution order, deep-merge overrides, region fallback (es-MX → es → en) and "registered locale replaces built-in" are unchanged. registerLocale invalidates the caches.
• all.ts lists every group and backs Messages, public getMessages() and useT() (full catalog, bundled only if imported). I18nProvider, setLocale, registerLocale, useLocale unchanged.
• scripts/size-lib.ts: bundling helpers extracted from size-limit.ts so tests reuse them.
• AGENTS.md i18n section and the i18n docs page updated (new authoring rule); changeset: minor.

Size (gzip, esbuild min, Svelte runtime included)

entry	before	after	limit before → after
floor: Divider only	17.57 kB	17.57 kB	18.42 → 18.42 kB
root: Button only	27.89 kB	23.91 kB (-3.98)	27.93 → 25.11 kB
root: AppShellLayout	39.88 kB	36.07 kB (-3.81)	39.94 → 37.88 kB
./seo	30.01 kB	30.01 kB	unchanged
styles/*.css	unchanged	unchanged	unchanged

Before: dist/i18n/pt-BR.js + en.js were 13.1 kB minified (uncompressed) of Button's bundle.

Test-first (tests/i18n-split.test.ts)

Before the change:

(fail) Button-only bundle contains none of the other components' strings
- [] + ["Command palette","Paleta de comandos","Search failed","Selected values","Valores selecionados","Module navigation","Resize navigation panel","Replay","On this page","Could not copy"]
(fail) a component brings its own group, not its neighbours'   (Expected not to contain "Module navigation")
 1 pass / 2 fail

After: 10 pass 0 fail (i18n-split + i18n + i18n-register).

Verification

• bun run test:browser: 154 files, 1555 tests pass
• bun run test:unit: 824 pass; test:ssr: 3 pass
• bun run validate: ok (build, themes, token lint, biome, types package+site, unit, size)

[github-actions]

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