refactor: move SIGMA_SB constant from solver.py to constants.py

Follow project convention "No magic numbers: Physical constants live
in constants.py". The Stefan-Boltzmann constant was defined locally
in solver.py — now centralized alongside other physical constants.

https://claude.ai/code/session_018WB9yHTAM3NZVpids3fo8m

Author: claude

src/venting/constants.py

@@ -20,3 +20,6 @@
 
 PI_C = (2.0 / (GAMMA + 1.0)) ** (GAMMA / (GAMMA - 1.0))
 C_CHOKED = math.sqrt(GAMMA * (2.0 / (GAMMA + 1.0)) ** ((GAMMA + 1.0) / (GAMMA - 1.0)))
+
+# Stefan-Boltzmann constant [W/(m^2 K^4)]
+SIGMA_SB = 5.670374419e-8

src/venting/solver.py

@@ -4,7 +4,18 @@
 from scipy.integrate import solve_ivp
 
 from .cases import CaseConfig
-from .constants import C_CHOKED, C_P, C_V, GAMMA, M_SAFE, P0, R_GAS, T0, T_SAFE
+from .constants import (
+    C_CHOKED,
+    C_P,
+    C_V,
+    GAMMA,
+    M_SAFE,
+    P0,
+    R_GAS,
+    SIGMA_SB,
+    T0,
+    T_SAFE,
+)
 from .flow import (
     mdot_fanno_tube,
     mdot_orifice_pos_props,
@@ -21,8 +32,6 @@
 )
 from .thermo import cp_air, cv_air, gamma_air, h_air, u_air
 
-SIGMA_SB = 5.670374419e-8
-
 
 def _mdot_short_edge(
     edge: ShortTubeEdge,