_pe_f64igammaq.nx

_pe_f64igammaq.nx -- integer-order regularized upper incomplete gamma (DLMF 8.4.7, the Poisson/Erlang/chi-square CDF): Q(n,x) = Gamma(n,x)/Gamma(n) = e^(-x) * SUM_{k=0}^{n-1} x^k / k! (n >= 1) P(n,x) = 1 - Q(n,x) CLOSED FORM (finite sum) for integer order -- no series-vs-CF regime split, all terms positive, self-contained (only nx_f64_exp + f64 arithmetic). term_k = term_{k-1}*x/k. f64 software-IEEE throughout. license_tier: ORIGINAL module: nishi-core.math.f64_igammaq depends: nishi-core.math.f64, nishi-core.math.f64_exp capability: F64_INCOMPLETE_GAMMA_INTEGER