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

src/cryo_tank/config.py

@@ -5,13 +5,6 @@ Pure data module -- no functions, no side effects.
 """
 import math
 
-# ---------- Gas constants ----------
-R_UNIVERSAL = 8314.46       # J/(kmol*K)
-M_N2 = 28.014              # kg/kmol
-M_HE = 4.0026              # kg/kmol
-R_HE = R_UNIVERSAL / M_HE  # 2077.1 J/(kg*K)
-R_N2 = R_UNIVERSAL / M_N2  # 296.8 J/(kg*K)
-
 # ---------- Tank geometry ----------
 V_TOTAL = 420.1e-3          # m^3  (420.1 L)
 H_TANK = 0.5                # m    (cylinder height)