Merge `_newton_girard` and `_newton_girard_matrices` (OAK/Sobol)

[thomaspinder]

Category: simplify · Severity: medium
Location: gpjax/kernels/additive/oak.py:60 · gpjax/kernels/additive/sobol.py:123

Problem

_newton_girard (oak.py:60) and _newton_girard_matrices (sobol.py:123) are the same algorithm — identical power-sum, sign, mask, and fori_loop structure — differing only by trailing broadcast dims of size 1. sobol's own docstring says it is 'the matrix-level analogue of _newton_girard in oak.py'.

Recommendation

Merge into one broadcast-generic implementation and call it from both sites.

_{Filed from the 2026-07-02 GPJax codebase audit (medium tier). Companion spec for the high-severity items: plans/2026-07-02-audit-high-severity-remediation-design.md.}