Previously, both missing files (fs.statSync throws) and 0-byte files
produced the same error "Datei ist leer (0 Bytes)". Now:
- Missing files: "Datei nicht gefunden"
- Empty files: "Datei ist leer (0 Bytes)"
Also adds 3 edge case tests (throttle consume(0), unlimited rate,
semaphore release-without-acquire). All 66 tests passing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>