Split-prime supercongruences at a mixed CM point: order drop, Eisenstein dichotomy, and reduction to a bivariate jet congruence

We prove the order drop, modular realization, and split/inert Eisenstein dichotomy for symmetric-cube hypergeometric coefficients at a mixed CM point. At split primes both Eisenstein seeds share the same Hecke eigenvalue and the tower survives; at inert primes a unit-root local factor obstructs it. The split-prime supercongruence is reduced to an explicit bivariate Fricke–Hecke congruence on a two-dimensional Eisenstein module via a logarithmic jet-extension framework, in which transport layers vanish by a binomial cancellation rather than by size estimates.