Port FluidSim core and scenarios to C++20

include/FluidSim/Constants.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+namespace fluidsim::port {
+
+inline constexpr float pi = 3.14159265358979323846f;
+inline constexpr float gamma = 1.4f;
+// PipeSystem.cs stores these as independent float constants rather than
+// deriving them from Gamma at runtime; preserve that rounding in the port.
+inline constexpr float pipe_gm1 = 0.4f;
+inline constexpr float pipe_gm1_inverse = 1.0f / pipe_gm1;
+inline constexpr float gas_constant = 287.0f;
+inline constexpr float ambient_pressure = 101325.0f;
+inline constexpr float ambient_temperature = 300.0f;
+inline constexpr float ambient_density = ambient_pressure / (gas_constant * ambient_temperature);
+
+} // namespace fluidsim::port