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/cli/options.cpp

@@ -110,6 +110,9 @@ Options parseOptions(int argc, char** argv) {
                     }
                     break;
                 }
+                case ParamSpec::Type::Path:
+                    options.params[spec->key] = requireValue(argc, argv, i, arg);
+                    break;
                 case ParamSpec::Type::Choice: {
                     const std::string text = requireValue(argc, argv, i, arg);
                     bool valid = false;