feat(l10n): single-source localization pipeline

Add localization/ — one strings.json is the source of truth for every
user-facing string, and a Bun CLI generates the platform files:

  loc migrate   rebuild strings.json from existing platform files (one-time)
  loc validate  structural checks (plural `other`, arg refs, coverage)
  loc generate  emit .xcstrings (Apple) + per-language strings.xml (KMP)

Canonical `{name}` placeholders are converted to each platform's positional
printf tokens using declared arg types. Plurals use CLDR categories and emit
native forms: xcstrings plural variations and Compose <plurals> blocks.

Migration folds the KMP `transfer_file_count_one`/`_other` pair into a single
plural key, so `transferFileCountResource` now returns the plural resource and
callers resolve it with pluralStringResource.

localization/src/commands/validate.ts

@@ -0,0 +1,55 @@
+/** Structural checks on strings.json. Exits non-zero on errors (warnings don't fail). */
+import { STRINGS_JSON } from "../config";
+import type { StringsFile } from "../types";
+
+const TOKEN = /\{([A-Za-z_][A-Za-z0-9_]*)\}/g;
+
+export async function validate() {
+  const doc = JSON.parse(await Bun.file(STRINGS_JSON).text()) as StringsFile;
+  const errors: string[] = [];
+  const warnings: string[] = [];
+
+  for (const [key, entry] of Object.entries(doc.strings)) {
+    const where = `"${key}"`;
+    const argNames = new Set((entry.args ?? []).map((a) => a.name));
+
+    if (entry.translations && entry.plural) {
+      errors.push(`${where}: has both "translations" and "plural" (pick one).`);
+    }
+    if (!entry.translations && !entry.plural) {
+      warnings.push(`${where}: no translations yet.`);
+    }
+    if (!entry.context?.trim() || entry.context.startsWith("TODO")) {
+      warnings.push(`${where}: missing/placeholder context.`);
+    }
+
+    const texts: string[] = [];
+    if (entry.translations) texts.push(...Object.values(entry.translations));
+    if (entry.plural) {
+      for (const [lang, forms] of Object.entries(entry.plural)) {
+        if (!forms.other?.trim()) errors.push(`${where} [${lang}]: plural missing required "other".`);
+        texts.push(...Object.values(forms));
+      }
+    }
+
+    for (const text of texts) {
+      for (const m of text.matchAll(TOKEN)) {
+        if (!argNames.has(m[1]!)) {
+          errors.push(`${where}: uses {${m[1]}} but no matching arg is declared.`);
+        }
+      }
+    }
+
+    // Missing-language coverage (warn only).
+    for (const lang of doc.supportedLanguages) {
+      const present = entry.plural ? !!entry.plural[lang] : entry.translations?.[lang] !== undefined;
+      const anyTranslations = entry.translations || entry.plural;
+      if (anyTranslations && !present) warnings.push(`${where}: missing "${lang}".`);
+    }
+  }
+
+  for (const w of warnings) console.warn(`warn  ${w}`);
+  for (const e of errors) console.error(`error ${e}`);
+  console.log(`\n${Object.keys(doc.strings).length} keys — ${errors.length} error(s), ${warnings.length} warning(s).`);
+  if (errors.length) process.exit(1);
+}