the non-relativistic H_core = T + V with the scalar relativistic H^DK.
The DK transformation is carried out in the momentum (p) representation
obtained by diagonalising the kinetic energy matrix T.
Pipeline (called once per SCF):
1. Compute pVp integrals
2. Build p-space basis: S^{-1/2} -> XU, SXU, p^2 eigenvalues
3. Compute kinematic factors: E_p, A, R
4. Transform V and pVp to p-space
5. Build H^DK1 in p-space (DK1 correction)
6. Add H^DK2 correction in p-space (DK2 correction)
7. Back-transform to AO basis -> overwrite OQP::Hcore