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

Implemented as a composite of engine primitives so autograd flows through naturally, and its gradient is the trigamma function. For x >= 0 the argument is shifted by 8 via $\psi(x) = \psi(x + 8) - \sum_{k=0}^{7} 1/(x + k)$ and the asymptotic Bernoulli series is evaluated there; for x < 0 the reflection formula $\psi(x) = \psi(1 - x) - \pi \cot(\pi x)$ maps the argument onto the right half.