Fix frozen indices

[markleader]

Summary

Fix frozen rocket scheduling when n_frz falls inside a multi-point exit schedule, restoring parity with legacy CEA2 for n_frz=3 and higher. Closes #95 .

Also add experimental n_frz=-1 support to expand the supplied reactant composition through the nozzle without performing any equilibrium solves.

Changes

• Split IAC and FAC pi_p and supar schedules into:

  • An equilibrium prefix through the freeze station.
  • A frozen suffix after the freeze station.

• Preserve the freeze-station composition at every downstream frozen point.

• Add an experimental n_frz=-1 mode that:

  • Initializes the chamber state directly from the reactants.
  • Supports both temperature and enthalpy inputs.
  • Bypasses all EqSolver.solve calls.
  • Uses the existing frozen-property kernels for IAC and FAC throat and exit calculations.
  • Supports pi_p, subar, and supar schedules.
  • Passes through the existing Fortran, C, Python, MATLAB, Excel, and command-line interfaces without restricting n_frz to positive values.

• Prevent negative composition indexing in command-line frozen output.

Testing

• Added one pFUnit regression test reproducing the RP-1/O₂/CO₂/H₂O n_frz=3 issue.
• The regression test verifies:
  • All four stations complete and converge.
  • Station 4 retains station 3’s species amounts.
  • Temperature, thrust coefficient, specific impulse, vacuum specific impulse, and characteristic velocity agree with local legacy CEA2 results to approximately 1e-5 relative tolerance.

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)

Numerical results change for freeze boundaries that previously occurred inside a multi-point pi_p or supar schedule. These cases now transition from equilibrium to frozen composition at the requested station and match legacy CEA2 behavior.

n_frz=-1 introduces new experimental numerical behavior with no CEA2 equivalent: reactants are expanded with fixed composition and no equilibrium solves.