- Add FileCount, FileSize, RelFilePath, AbsFilePath, ModTime, Multihash types
- Add UnixSeconds and UnixNanos types for timestamp handling
- Add URL types (ManifestURL, FileURL, BaseURL) with safe path joining
- Consolidate scanner package into mfer package
- Update checker to use custom types in Result and CheckStatus
- Add ModTime.Timestamp() method for protobuf conversion
- Update all tests to use proper custom types
