Add an image overlay effect

Lays a single picture from disk over its input: fitted, filled, stretched,
left at its own size or tiled, centred or pinned to the top left corner,
nudged by an offset and mixed in by its opacity.

The file it needs is a new kind of param, Path, so the interface shows a
chooser and the CLI takes a file name, both without knowing an overlay
exists. Projects write it relative to their own folder, as they do a
source, and Image::load can now hand back the alpha channel instead of
flattening it onto white, so a cut-out PNG keeps its cut-out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/registry.cpp

@@ -3,6 +3,7 @@
 #include "effects/blur.hpp"
 #include "effects/grain.hpp"
 #include "effects/monochrome.hpp"
+#include "effects/overlay.hpp"
 #include "effects/scanlines.hpp"
 
 namespace effector {
@@ -14,6 +15,7 @@ const std::vector<EffectPtr>& effects() {
         std::make_shared<BlurEffect>(),
         std::make_shared<GrainEffect>(),
         std::make_shared<MonochromeEffect>(),
+        std::make_shared<OverlayEffect>(),
     };
     return registry;
 }