PlotAzintUnit used D_A while MultiLinePlotUnits used d_A for the same
d-spacing-in-angstrom unit. Align PlotAzintUnit to the lowercase crystallographic
convention already used by MultiLinePlotUnits. Generated Plot_unit_x::D_A is
left as-is (controlled by the OpenAPI generator's C++ enum casing).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>