feat: complete image phase - SHA-256 exact + dHash perceptual duplicate detection

- lib.rs: scan_images, compute_dhash, hamming, find_duplicate_groups
- main.rs: CLI with folder arg and optional hamming threshold
- 13 unit tests: hamming, is_image_path, dhash, find_duplicate_groups
- 7 integration tests: real files, empty dir, cropped, non-image exclusion,
  subdirectory recursion, single file, CLI binary output
- All 20 tests passing

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