Major changes:
- Refactor CLI to accept injected I/O streams and filesystem (afero.Fs)
for testing without touching the real filesystem
- Add RunOptions struct and RunWithOptions() for configurable CLI execution
- Add internal/scanner package with two-phase manifest generation:
- Phase 1 (Enumeration): walk directories, collect metadata
- Phase 2 (Scan): read contents, compute hashes, write manifest
- Add internal/checker package for manifest verification with progress
reporting and channel-based result streaming
- Add mfer/builder.go for incremental manifest construction
- Add --no-extra-files flag to check command to detect files not in manifest
- Add timing summaries showing file count, size, elapsed time, and throughput
- Add comprehensive tests using afero.MemMapFs (no real filesystem access)
- Add contrib/usage.sh integration test script
- Fix banner ASCII art alignment (consistent spacing)
- Fix verbosity levels so summaries display at default log level
- Update internal/log to support configurable output writers
