Split-prime supercongruences at a mixed CM point: order drop, Eisenstein dichotomy, and unconditional mod p² closure

We study symmetric-cube hypergeometric coefficients at the mixed CM point (1/6, 1/3;1). The full split-prime supercongruence A (mp) ≡ A (m) (mod p⁴) is reduced to a single scalar congruence S (p) ≡ 27 (mod p⁴). This sum is evaluated unconditionally modulo p and modulo p², the latter via a companion endpoint theorem Ã₏−₁ ≡ 2·27^p−1−1 (mod p²) proved using the Eisenstein–Lerch formula. The mod p³ layer is reduced to one explicit weighted congruence but remains open. All other results — order drop, Eisenstein dichotomy, Hecke–Fricke finite reduction, module-wide failure, and self-reciprocity pairing — are unconditional.