[milanofthe]

Fixes #22.

• Gas phase scaled by the equilibrium ratio (w = y_T2 / nu), so all states are of order one at any inlet concentration.
• Gas-limited regime (k >= 1e3, k ~ 1/c_T_in): closed-form leading-order limit for both BCs, agreeing with the solve to 3.2/k. A tritium-free inlet gives zero exactly.
• Initial mesh graded towards the gas inlet and outlet layers, and a lower bound on the scaled gas inlet, which removes the square-root singularity at a T2-free inlet under O-C.
• g argument used (from steps-re), flow_g documented as molar flow, parameter and input validation, NaN passed through.
• Requires pathsim 0.30.3 (cold restart after a failed warm start, graded initial mesh).