Fix downstream shock Mach numbers

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Summary

Correct downstream Mach outputs in the Fortran shock solver.

Changes

• Calculate Mach from finalized velocity and sound speed across equilibrium, frozen, and retained-last-valid paths.
• Document shock reference frames and sound-speed conventions.
• Preserve M21/M52 as molecular-weight ratios and correct misleading documentation.
• Add physical-validation, input-mode, reuse, and failure regressions.

Testing

• Rebuilt the Fortran library and Python extension.
• All 15 CTest targets and 99 Python tests passed.
• Validated incident/reflected shocks against monatomic-gas normal-shock relations.
• All 944 captured non-Mach fields remained bitwise unchanged; legacy example output remained byte-identical.

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes: downstream Mach values are corrected. The reproducer now returns [4.171573075415, 0.689598550089, 0.662949797119]. Thermochemical states, sound-speed conventions, and legacy molecular-weight ratios remain unchanged.