Checkpoint 1: implement native subsystems and begin the gameplay manual

include/faset/authoring/templates.hpp

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+#pragma once
+#include <faset/authoring/schema.hpp>
+#include <functional>
+#include <string>
+
+namespace faset::authoring {
+struct ResolvedScene {Json scene;Json conflicts=Json::array();};
+using SceneLoader=std::function<Json(const std::string&)>;
+// Source documents are immutable inputs. Conflicting records stay in the authoring file.
+ResolvedScene resolve_templates(const Json& scene,const SchemaRegistry& schemas,const SceneLoader& loader);
+}