Update file permission literals from legacy octal format (0755, 0644) to explicit Go 1.13+ format (0o755, 0o644) for improved readability.
- Add explicit error ignoring with _ = for Close/Remove calls - Rename WriteTo to Write to avoid io.WriterTo interface conflict - Fix errcheck warnings in fetch, freshen, gen, mfer, checker, deserialize, serialize, and output files
Replace callback-based progress reporting in Builder.AddFile with channel-based FileHashProgress for consistency with EnumerateStatus and ScanStatus patterns. Update scanner.go to use the new channel API.
