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

Cargo.toml

@@ -4,3 +4,7 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+image = { version = "0.25", default-features = true, features = ["jpeg", "png", "gif", "webp", "bmp", "tiff"] }
+sha2 = "0.10"
+walkdir = "2.5"
+anyhow = "1"