quiver: Marker & arrowhead name/value property style consistency (bug #64134) * __quiver__.m: Add have_line_spec check after applying any name-value properties to maintain arrowhead display status. * __quiver__.m (update_plot): Add linewidth property to base marker object set call. * quiver.m: Add BIST to ensure marker properties are updated correctly. Remove unnecessary strcmp from other BIST assert calls. * etc/NEWS.9.md: Update description of changes to quiver note under General Improvements.