Maps vertices, flags, justification, normal, scale and style with host
validation and MLineStyle binding. The host recomputes vertex direction,
miter and element parameters on scripted create/edit, so scripts only supply
positions. staged_python_mline_lifecycle_over_real_ipc passes the full gate
including DWG and DXF reopen.
Co-Authored-By: Claude Sonnet 5 <[email protected]>