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/lib/xcstrings.ts

@@ -0,0 +1,45 @@
+/** Read/write Apple String Catalog (.xcstrings) — a single JSON file, all languages nested. */
+
+export type XcState = "new" | "translated" | "needs_review" | "stale";
+
+export interface XcStringUnit {
+  state: XcState;
+  value: string;
+}
+export interface XcVariations {
+  plural?: Record<string, { stringUnit: XcStringUnit }>;
+}
+export interface XcLocalization {
+  stringUnit?: XcStringUnit;
+  variations?: XcVariations;
+}
+export interface XcEntry {
+  comment?: string;
+  extractionState?: string;
+  localizations?: Record<string, XcLocalization>;
+}
+export interface XcCatalog {
+  sourceLanguage: string;
+  strings: Record<string, XcEntry>;
+  version: string;
+}
+
+export function parseXcstrings(json: string): XcCatalog {
+  return JSON.parse(json) as XcCatalog;
+}
+
+/** Stable, Xcode-compatible serialization: 2-space indent, keys sorted, trailing newline. */
+export function renderXcstrings(cat: XcCatalog): string {
+  const sortDeep = (v: unknown): unknown => {
+    if (Array.isArray(v)) return v.map(sortDeep);
+    if (v && typeof v === "object") {
+      const out: Record<string, unknown> = {};
+      for (const k of Object.keys(v as Record<string, unknown>).sort()) {
+        out[k] = sortDeep((v as Record<string, unknown>)[k]);
+      }
+      return out;
+    }
+    return v;
+  };
+  return JSON.stringify(sortDeep(cat), null, 2) + "\n";
+}