Digamma function $\psi(x) = \frac{d}{dx} \ln \Gamma(x)$.

Implemented as a composite of engine primitives so autograd flows through naturally. Uses the standard recurrence-plus-asymptotic strategy: shift the argument by 8 via $\psi(x) = \psi(x + 8) - \sum_{k=0}^{7} 1/(x + k)$, then evaluate the asymptotic Bernoulli series on the shifted argument where the series converges quickly.