Add checksum verification feature

[felix068]

Implements feature request #71 for optional checksum verification during file copies to detect hardware errors (memory/storage issues).

Implementation:

• Uses xxHash64 for fast non-cryptographic checksumming
• Calculates checksum during copy (zero overhead on source read)
• Verifies by re-reading destination file only
• Returns immediate error on mismatch (no retry)
• Works with both parfile and parblock drivers
• Thread-safe using Mutex for final checksum storage

Changes:

• Add --verify-checksum CLI flag
• Add Config.verify_checksum field
• Add XcpError::ChecksumMismatch error type
• Update CopyHandle to compute and verify checksums
• Add xxhash-rust dependency

Tests:

• 28 comprehensive test cases covering:
  • Empty, small, and large files
  • Binary patterns (zeros, random, alternating)
  • Recursive directory copies
  • Multiple files
  • Both drivers (parfile/parblock)
  • Various block sizes and worker counts
  • Sparse files
  • File overwriting

Performance:

• ~2x overhead due to destination re-read (e.g., 34ms → 70ms for 50MB)
• Acceptable trade-off for critical data integrity

Addresses feedback from @tarka, @Kalinda-Myriad, and @OndrikB in issue #71.