fix(cryo_tank): use helium-only CoolProp pressure model

tests/cryo_tank/test_properties.py

@@ -6,7 +6,6 @@ sys.path.insert(0, "src")
 
 from cryo_tank.properties import (
     ln2_rho, ln2_h, ln2_u, ln2_T_from_u,
-    n2_vapor_u, n2_sat_pressure,
     he_u, he_h, he_cp, he_cv,
 )
 from cryo_tank.config import P_WORKING
@@ -34,24 +33,12 @@ class TestLN2Properties:
         assert abs(T_recovered - T_orig) < 0.01
 
 
-class TestN2VaporProperties:
-    """N2 vapor properties."""
-
-    def test_n2_sat_pressure_at_78K(self):
-        P_sat = n2_sat_pressure(78.0)
-        assert 0.10e6 < P_sat < 0.12e6  # ~0.1093 MPa
-
-    def test_n2_vapor_internal_energy_at_78K(self):
-        u = n2_vapor_u(78.0)
-        assert 50000 < u < 60000  # ~55547 J/kg
-
-
 class TestHeliumProperties:
-    """Helium (ideal gas) properties."""
+    """Helium properties."""
 
     def test_he_cp_near_5196(self):
         cp = he_cp()
-        assert abs(cp - 5196.2) < 10  # monatomic ideal gas
+        assert abs(cp - 5196.2) < 10
 
     def test_he_cv_near_3117(self):
         cv = he_cv()

tests/cryo_tank/test_tank_model.py

@@ -69,10 +69,9 @@ class TestInitialState:
         assert abs(info['T_liq'] - T_INIT) < 0.1
         assert abs(info['T_ull'] - T_INIT) < 1.0
 
-    def test_initial_pressure_components_sum_to_P_working(self):
+    def test_initial_pressure_is_provided_entirely_by_helium(self):
         tank = _make_tank()
         y0 = tank.initial_state()
         info = tank.derive(y0)
-        P_N2 = info['P_N2']
         P_He = info['P_He']
-        assert abs(P_N2 + P_He - P_WORKING) / P_WORKING < 1e-6
+        assert abs(P_He - P_WORKING) / P_WORKING < 1e-12