Checkpoint 1: implement native subsystems and begin the gameplay manual

include/faset/core/io.hpp

@@ -0,0 +1,15 @@
+#pragma once
+#include <faset/core/json.hpp>
+#include <filesystem>
+#include <string>
+#include <string_view>
+
+namespace faset {
+std::string new_id();
+std::string read_text(const std::filesystem::path& path);
+Json read_json(const std::filesystem::path& path);
+void atomic_write(const std::filesystem::path& path, std::string_view bytes);
+void atomic_write_json(const std::filesystem::path& path, const Json& value);
+// Rejects traversal and symlink escapes before project-scoped file operations.
+std::filesystem::path project_path(const std::filesystem::path& root, const std::filesystem::path& relative);
+}