New Operator: AngularMomentum

This one is just about as fundamental as the Laplacian. For complex spherical harmonics and Zernike polynomials, it's diagonal. Other multivariate OPs may have nontrivial representations. In polar coordinates, it's ∂/∂θ; in spherical coordinates, it's ∂/∂φ. Or in both systems, it's x ∂/∂y - y ∂/∂x. Since it's spectrum "separates orders," it is complementary to the Laplacian which "separates degrees."

No files, tests, or entry points are named. Start by locating the existing Laplacian and other differential-operator implementations for multivariate orthogonal polynomials, then determine how an AngularMomentum operator would represent ∂/∂θ, ∂/∂φ, or x ∂/∂y - y ∂/∂x across supported domains; done means the operator is implemented and covered for the stated polynomial families.