Port FluidSim core and scenarios to C++20

include/FluidSim/Atmosphere.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include "FluidSim/Engine.hpp"
+
+namespace fluidsim::port {
+
+// Fixed thermodynamic boundary matching the C# Atmosphere component.
+class Atmosphere final : public Component {
+public:
+    float pressure{101325.0f};
+    float temperature{300.0f};
+    float gas_constant_value{gas_constant};
+    float gamma_value{gamma};
+    Port port{};
+
+    Atmosphere() { update_port(); }
+
+    float density() const { return pressure / (gas_constant_value * temperature); }
+    float specific_enthalpy() const {
+        return gamma_value / (gamma_value - 1.0f) * pressure / density();
+    }
+    std::vector<Port*> ports() override { return {&port}; }
+    void update_state(float /*dt*/) override { update_port(); }
+
+private:
+    void update_port() {
+        port.pressure = pressure;
+        port.density = density();
+        port.temperature = temperature;
+        port.enthalpy = specific_enthalpy();
+        port.air_fraction = 1.0f;
+    }
+};
+
+} // namespace fluidsim::port