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/lib.rs

@@ -1,5 +1,7 @@
 pub mod config;
+pub mod discovery;
 pub mod error;
+pub mod loader;
 pub mod operations;
 pub mod pipeline;
 pub mod preset;