linalg: Jacobi eigenvalues for symmetric matrices

Add numx_mat_eigenvalues_sym (or similar name) to the linalg module: the Jacobi eigenvalue algorithm for real symmetric matrices, bounded by NUMX_MAX_MAT_ROWS.

Follow CONTRIBUTING.md exactly: full Doxygen header, NULL-checks, numx_status_t return, all four deliverables (header, impl, test, docs page). Test against known eigenvalue examples and verify the trace/determinant invariants hold against the computed eigenvalues.