fix: provide assoc_legendre fallback for libc++ (macOS build)

[manishpaulish]

spherical_harmonic.cpp uses std::assoc_legendre, one of the C++17 mathematical special functions. These are optional in the standard, and libc++ (the default standard library on macOS) does not implement them, so the build currently fails on macOS with:

error: no member named 'assoc_legendre' in namespace 'std'

This replaces the two std::assoc_legendre call sites with a small standalone implementation using the standard upward recurrence. The convention matches std::assoc_legendre exactly (no Condon-Shortley phase). I verified the output is bit-identical to libstdc++'s std::assoc_legendre across a range of (l, m, x) values.

The affected functions (sph_harm / real_sph_harm) are reference routines not on the production sphericart path, so there is no performance impact. Verified that libsymmetrix now builds cleanly on macOS (Apple Clang) with the Kokkos serial backend.