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/loader_tests.rs

@@ -0,0 +1,73 @@
+use pixstrip_core::loader::ImageLoader;
+use pixstrip_core::types::Dimensions;
+use std::path::Path;
+
+fn create_test_jpeg(path: &Path) {
+    use image::{RgbImage, ImageFormat};
+    let img = RgbImage::from_fn(100, 80, |x, y| {
+        image::Rgb([(x % 256) as u8, (y % 256) as u8, 128])
+    });
+    img.save_with_format(path, ImageFormat::Jpeg).unwrap();
+}
+
+fn create_test_png(path: &Path) {
+    use image::{RgbaImage, ImageFormat};
+    let img = RgbaImage::from_fn(200, 150, |x, y| {
+        image::Rgba([(x % 256) as u8, (y % 256) as u8, 100, 255])
+    });
+    img.save_with_format(path, ImageFormat::Png).unwrap();
+}
+
+#[test]
+fn load_jpeg_dimensions() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("test.jpg");
+    create_test_jpeg(&path);
+
+    let loader = ImageLoader::new();
+    let info = loader.load_info(&path).unwrap();
+    assert_eq!(info.dimensions, Dimensions { width: 100, height: 80 });
+    assert_eq!(info.format, Some(pixstrip_core::types::ImageFormat::Jpeg));
+}
+
+#[test]
+fn load_png_dimensions() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("test.png");
+    create_test_png(&path);
+
+    let loader = ImageLoader::new();
+    let info = loader.load_info(&path).unwrap();
+    assert_eq!(info.dimensions, Dimensions { width: 200, height: 150 });
+    assert_eq!(info.format, Some(pixstrip_core::types::ImageFormat::Png));
+}
+
+#[test]
+fn load_pixels() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("test.jpg");
+    create_test_jpeg(&path);
+
+    let loader = ImageLoader::new();
+    let pixels = loader.load_pixels(&path).unwrap();
+    assert_eq!(pixels.width(), 100);
+    assert_eq!(pixels.height(), 80);
+}
+
+#[test]
+fn load_nonexistent_file() {
+    let loader = ImageLoader::new();
+    let result = loader.load_info(Path::new("/tmp/does_not_exist_pixstrip.jpg"));
+    assert!(result.is_err());
+}
+
+#[test]
+fn file_size_bytes() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("test.jpg");
+    create_test_jpeg(&path);
+
+    let loader = ImageLoader::new();
+    let info = loader.load_info(&path).unwrap();
+    assert!(info.file_size > 0);
+}