高温氦气物性补全；三通四通能量计算bug修正

app/simulation/components/amesim/media/mediums.py

@@ -20,6 +20,12 @@ class AmesimIdealAirMedium(IdealGasMedium):
 
 @dataclass(frozen=True)
 class AmesimHeliumPengRobinsonMedium(IdealGasMedium):
+    """Helium identity; reference constants below describe the lower range.
+
+    The native evaluator uses Amesim's piecewise NASA heat capacity/enthalpy
+    (6000 K transition) and viscosity (1000/5000 K transitions), together
+    with PR departure properties. ``cp_ref`` is not a global constant Cp.
+    """
     SUBSTANCE_ID: ClassVar[str] = 'helium'
     PROPERTY_METHOD_ID: ClassVar[str] = 'peng_robinson'
     nasa_cp_over_R: ClassVar[float] = 2.5