Implemented the real server

src/game_server/GameServerRouter.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include "IGameServer.h"
+#include <stdexcept>
+
+namespace FRG {
+
+// Keeps references held by UI and runtime objects stable while campaigns switch
+// between the local and remote implementations.
+class GameServerRouter final : public IGameServer {
+public:
+	explicit GameServerRouter(IGameServer& initial)
+		: active_(&initial)
+	{
+	}
+
+	void use(IGameServer& server) { active_ = &server; }
+	std::shared_ptr<const GameSnapshot> snapshot() const override {
+		return active_->snapshot();
+	}
+	const std::string& localPlayerId() const override {
+		return active_->localPlayerId();
+	}
+	void submit(const GameCommand& command) override { active_->submit(command); }
+	std::vector<GameServerEvent> drainEvents() override {
+		return active_->drainEvents();
+	}
+	void update(std::int64_t deltaMs) override { active_->update(deltaMs); }
+	void save(nlohmann::json& out) const override { active_->save(out); }
+	void load(const nlohmann::json& in) override { active_->load(in); }
+
+private:
+	IGameServer* active_;
+};
+
+} // namespace FRG