feat: detect image format from magic bytes, not file extension

- Fixes misnamed files (e.g. JPEG saved as .png) being skipped
- Uses image::ImageReader with guessed format from content
- Fixes Android screenshots with wrong extension being skipped
- New test: misnamed_jpeg_as_png_still_scanned
- 22 tests passing

tests/image_phase.rs

@@ -133,3 +133,16 @@ fn cli_binary_reports_duplicates() {
     assert!(stdout.contains("[exact]"), "output should contain exact groups: {stdout}");
     assert!(stdout.contains("[similar]"), "output should contain similar groups: {stdout}");
 }
+
+#[test]
+fn misnamed_jpeg_as_png_still_scanned() {
+    // fake_png.png is actually JPEG data with .png extension
+    let dir = Path::new("/a0/usr/projects/deduper/.a0proj/test_media/images");
+    let entries = scan_images(dir).expect("scan");
+    let fake = entries.iter().find(|e| e.path.ends_with("fake_png.png"));
+    assert!(fake.is_some(), "misnamed JPEG-as-PNG should be scanned via magic bytes");
+    // should have same hash as orig.jpg since it's a copy
+    let orig = entries.iter().find(|e| e.path.ends_with("orig.jpg")).unwrap();
+    let fake = fake.unwrap();
+    assert_eq!(orig.sha256, fake.sha256, "same content = same sha256");
+}