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

src/main.rs

@@ -1,3 +1,42 @@
+use deduper::{find_duplicate_groups, scan_images, DuplicateKind};
+use std::env;
+use std::path::Path;
+
 fn main() {
-    println!("Hello, world!");
+    let args: Vec<String> = env::args().collect();
+    if args.len() < 2 {
+        eprintln!("usage: deduper <folder> [hamming-threshold]");
+        std::process::exit(1);
+    }
+
+    let root = Path::new(&args[1]);
+    let threshold = args
+        .get(2)
+        .and_then(|s| s.parse::<u32>().ok())
+        .unwrap_or(8);
+
+    let entries = match scan_images(root) {
+        Ok(v) => v,
+        Err(e) => {
+            eprintln!("scan error: {e}");
+            std::process::exit(1);
+        }
+    };
+
+    let groups = find_duplicate_groups(&entries, threshold);
+    if groups.is_empty() {
+        println!("no image duplicates found");
+        return;
+    }
+
+    for (idx, group) in groups.iter().enumerate() {
+        let kind = match group.kind {
+            DuplicateKind::Exact => "exact",
+            DuplicateKind::Similar => "similar",
+        };
+        println!("group {} [{}]", idx + 1, kind);
+        for path in &group.paths {
+            println!("  {}", path.display());
+        }
+    }
 }