Fourth coverage batch — file-backed cache and i18n, each driven against a
throwaway temp directory:
- FileCacheTest: set/get roundtrip through serialization, miss = false, has(),
idempotent delete, flush, maxAge expiry (backdated mtime), and the path/age
accessors.
- TranslatorTest: English default, cookie language detection with fallback,
available() listing, {token} substitution, guarded setLanguage(), and the
hardened backfill — a key missing from the active language is written back
with the English value, or with itself as a visible placeholder when English
has none either.
+15 tests. Suite: 661 tests, 0 errors.
