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/gui/window.hpp

@@ -75,6 +75,10 @@ private:
     void updateTitle();
 
     void onParamChanged();
+    // The chooser behind a Path param. The chosen file goes straight into the
+    // stage's values rather than into the widget, which may well have been
+    // rebuilt by the time the dialog comes back.
+    void onChoosePath(int stage, const std::string& key, const std::string& label);
     void onSeedEntered();
     void onRecreate();
     void onSave();