Builds the SAP potential matrix V_{mu,nu} = sum_g w_g phi_mu(r_g) V_SAP(r_g) phi_nu(r_g), V_SAP(r) = sum_A V_A(|r - R_A|) = sum_A -Z_eff^A(|r - R_A|)/|r - R_A|, on the existing DFT molecular grid, reusing the AO-on-grid evaluation and AO-pruning machinery of mod_dft_gridint (run_grid_aos). The construction mirrors the Kohn-Sham matrix accumulation in mod_dft_gridint_energy.
Reference: S. Lehtola, "Assessment of Initial Guesses for Self-Consistent Field Calculations. Superposition of Atomic Potentials: Simple yet Efficient", J. Chem. Theory Comput. 15, 1593 (2019).