_bf_erfc_oracle_gate.nx

_bf_erfc_oracle_gate.nx -- ALGEBRAIC self-gate for the sovereign 120-bit erfc CF oracle (nx_bigfloat120_erfc, ME2-ERF-002 foundation). No external reference: the oracle proves ITSELF by the identity erf(x)+erfc(x)=1, checked at FULL bigfloat precision in the overlap window x in [1.5,1.75] where BOTH the 48-term erf Taylor AND the depth-240 erfc CF are accurate. Two INDEPENDENT methods agreeing to >2^-80 on the identity is conclusive proof both are correct (a correlated 80-bit error is nil). Plus a structural check over the TARGET domain x in [1.75,6]: erfc strictly decreasing and strictly positive (the property the f64 kernel will rely on). Durable: ERFCORACLE row -> knowledge/status/math_engine.log. Exit 0 iff both held. license_tier: ORIGINAL