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/src/discovery.rs

@@ -0,0 +1,57 @@
+use std::path::{Path, PathBuf};
+
+use walkdir::WalkDir;
+
+const IMAGE_EXTENSIONS: &[&str] = &[
+    "jpg", "jpeg", "png", "webp", "avif", "gif", "tiff", "tif", "bmp", "heic", "heif", "jxl",
+    "svg", "ico",
+];
+
+fn is_image_extension(ext: &str) -> bool {
+    IMAGE_EXTENSIONS.contains(&ext.to_lowercase().as_str())
+}
+
+pub fn discover_images(path: &Path, recursive: bool) -> Vec<PathBuf> {
+    if path.is_file() {
+        if let Some(ext) = path.extension().and_then(|e| e.to_str()) {
+            if is_image_extension(ext) {
+                return vec![path.to_path_buf()];
+            }
+        }
+        return Vec::new();
+    }
+
+    if !path.is_dir() {
+        return Vec::new();
+    }
+
+    let max_depth = if recursive { usize::MAX } else { 1 };
+
+    WalkDir::new(path)
+        .max_depth(max_depth)
+        .into_iter()
+        .filter_map(|entry| entry.ok())
+        .filter(|entry| entry.file_type().is_file())
+        .filter(|entry| {
+            entry
+                .path()
+                .extension()
+                .and_then(|ext| ext.to_str())
+                .is_some_and(is_image_extension)
+        })
+        .map(|entry| entry.into_path())
+        .collect()
+}
+
+pub fn has_subdirectories(path: &Path) -> bool {
+    if !path.is_dir() {
+        return false;
+    }
+    std::fs::read_dir(path)
+        .map(|entries| {
+            entries
+                .filter_map(|e| e.ok())
+                .any(|e| e.file_type().is_ok_and(|ft| ft.is_dir()))
+        })
+        .unwrap_or(false)
+}