Checkpoint 1: implement native subsystems and begin the gameplay manual

include/faset/core/error.hpp

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+#pragma once
+#include <faset/core/json.hpp>
+#include <stdexcept>
+#include <string>
+
+namespace faset {
+class Error : public std::runtime_error {
+public:
+    Error(std::string code, std::string message, Json details = Json::object())
+        : std::runtime_error(std::move(message)), code_(std::move(code)), details_(std::move(details)) {}
+    const std::string& code() const noexcept { return code_; }
+    Json json() const { return {{"code", code_}, {"message", what()}, {"details", details_}}; }
+private:
+    std::string code_;
+    Json details_;
+};
+inline void require(bool condition, const std::string& code, const std::string& message) {
+    if (!condition) throw Error(code, message);
+}
+}

include/faset/core/hash.hpp

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+#pragma once
+#include <cstddef>
+#include <filesystem>
+#include <span>
+#include <string>
+#include <string_view>
+
+namespace faset {
+std::string sha256(std::span<const std::byte> bytes);
+inline std::string sha256(std::string_view text) {
+    return sha256(std::as_bytes(std::span(text.data(), text.size())));
+}
+std::string sha256_file(const std::filesystem::path& path);
+}

include/faset/core/io.hpp

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+#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);
+}

include/faset/core/json.hpp

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+#pragma once
+#include <nlohmann/json.hpp>
+namespace faset { using Json = nlohmann::json; }