nx_mathspec_erfc.nx

nx_mathspec_erfc.nx -- SOVEREIGN SPEC EMITTER for the ERFC kernel (DLMF ch7 rung 2, ME2-ERF-002, the Laplace-CF range 1.75..6 that erf-Taylor cannot reach). The erfc CF kernel is control-flow (the backward recurrence), so its spec is just the scale/CF constants -- every one DERIVED from the 120-bit bigfloat, no python, no minimax: 1/sqrt(pi) = (2/sqrt(pi))/2 where 2/sqrt(pi) is bf_pi + Newton sqrt + bf_div (the SAME derivation the erf spec rail-proved bit-exact), halved by an exact exponent decrement. 0.5 and 1.0 are the exact CF coefficients (structural, exact f64). REFUSAL RAIL (refuse > fabricate): derived 1/sqrt(pi) bits MUST equal the known-good libm constant 0x3FE20DD750429B6D, else exit 2 no emit (derive sovereignly, verify vs the world). Layout: c[0]=inv_sqrt_pi c[1]=half(0.5) c[2]=one(1.0) Emits runtime/_hdl_build/_pm_erfc_spec.nx :: pm_erfc_spec_fill(c)->3 license_tier: ORIGINAL