perf(lib): stream download and extraction in single pass

- Replace two-phase download-then-extract with streaming pipeline
- Use io.TeeReader for concurrent SHA256 checksum calculation
- Eliminate temporary file storage (~100MB disk I/O saved)
- Add progress reporting with percentage and MB counters
- Fetch expected checksum before download for early verification
- Clean up extracted files on checksum mismatch or extraction error

Author: flexiondotorg