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/project.cpp

@@ -348,6 +348,13 @@ std::string projectToJson(const Project& project, const std::string& directory)
                 case ParamSpec::Type::Choice:
                     out << quote(paramChoice(stage.values, spec.key, spec.defaultChoice));
                     break;
+                case ParamSpec::Type::Path: {
+                    // Written relative to the project folder, exactly as the
+                    // sources are, so a folder still opens once it is moved.
+                    const auto path = paramText(stage.values, spec.key, "");
+                    out << quote(path.empty() ? path : relativeTo(path, directory));
+                    break;
+                }
             }
         }
         out << (specs.empty() ? "}" : "\n      }") << "\n    }";
@@ -418,6 +425,12 @@ Project projectFromJson(const std::string& text, const std::string& directory) {
                     case ParamSpec::Type::Double:
                         if (value->kind == Json::Kind::Number) stage.values[spec.key] = value->number;
                         break;
+                    case ParamSpec::Type::Path:
+                        if (value->kind == Json::Kind::String) {
+                            stage.values[spec.key] =
+                                value->text.empty() ? value->text : resolve(value->text, directory);
+                        }
+                        break;
                     case ParamSpec::Type::Choice:
                         for (const auto& choice : spec.choices) {
                             if (value->kind == Json::Kind::String && value->text == choice) {