Near-Degenerate Reversible 1:2 Resonance and Sextic Crossover in a (2, 3, 5, 7) Carnot Geodesic Flow

Preprint and reproducibility package for a near-degenerate reversible 1:2 resonance in a reduced (2,3,5,7) Carnot geodesic flow. The work identifies a quartic near-degeneracy in a fixed reversible normal-form chart, promotion of the sixth-order Hamiltonian in a distinguished near-diagonal sector, two distinct periodic-orbit action scales, a direct quartic-sextic crossover, and residual elliptic period-two cycles at exact resonance. The results are supported by exact-return computations, multiprecision monodromy auditing, continuation in the crossover variable, action and Floquet measurements, and an independent fifth-order critical return-jet re-extraction. The exactly quartic-degenerate limit sigma = 0 and the associated Stokes/splitting problem are outside the scope of this version.