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/effect.cpp

@@ -16,6 +16,7 @@ ParamValues Effect::defaults() const {
                 values[spec.key] = spec.defaultValue != 0.0;
                 break;
             case ParamSpec::Type::Choice:
+            case ParamSpec::Type::Path:
                 values[spec.key] = spec.defaultChoice;
                 break;
         }