nx_calc_integrate.nx

nx_calc_integrate.nx -- symbolic integration engine. Per the comparison commit's named LOSE_BIG improvement vs Mathematica: "ship symbolic Integrate + Series + Solve". This commit ships Integrate. Series and Solve are queued. Pattern-matched antiderivative rules (the standard textbook table). Returns a *Term that downstream proofs can verify by differentiating and checking the result equals the integrand (the Fundamental Theorem of Calculus check, doable via nx_calc_deriv + EQ check).