Add ImageLoader and file discovery modules

ImageLoader: load image info (dimensions, format, file size) and pixels.
Discovery: find image files by extension, flat or recursive, single file or directory.
All 9 tests passing.

pixstrip-core/tests/discovery_tests.rs

@@ -0,0 +1,52 @@
+use pixstrip_core::discovery::discover_images;
+use std::fs;
+
+fn create_test_tree(root: &std::path::Path) {
+    fs::create_dir_all(root.join("subdir")).unwrap();
+    fs::write(root.join("photo1.jpg"), b"fake jpeg").unwrap();
+    fs::write(root.join("photo2.png"), b"fake png").unwrap();
+    fs::write(root.join("readme.txt"), b"not an image").unwrap();
+    fs::write(root.join("subdir/nested.webp"), b"fake webp").unwrap();
+    fs::write(root.join("subdir/doc.pdf"), b"not an image").unwrap();
+}
+
+#[test]
+fn discover_flat() {
+    let dir = tempfile::tempdir().unwrap();
+    create_test_tree(dir.path());
+
+    let images = discover_images(dir.path(), false);
+    assert_eq!(images.len(), 2);
+    let names: Vec<String> = images
+        .iter()
+        .map(|p| p.file_name().unwrap().to_string_lossy().into_owned())
+        .collect();
+    assert!(names.contains(&"photo1.jpg".to_string()));
+    assert!(names.contains(&"photo2.png".to_string()));
+}
+
+#[test]
+fn discover_recursive() {
+    let dir = tempfile::tempdir().unwrap();
+    create_test_tree(dir.path());
+
+    let images = discover_images(dir.path(), true);
+    assert_eq!(images.len(), 3);
+}
+
+#[test]
+fn discover_empty_dir() {
+    let dir = tempfile::tempdir().unwrap();
+    let images = discover_images(dir.path(), true);
+    assert!(images.is_empty());
+}
+
+#[test]
+fn discover_single_file() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("single.jpg");
+    fs::write(&path, b"fake jpeg").unwrap();
+
+    let images = discover_images(&path, false);
+    assert_eq!(images.len(), 1);
+}