Added left-click Engineer boarding

I made left-click board eligible infantry, including Engineers, into carriers and garrison buildings. Shift-click still selects the target, and right-click remains available for repair.

A wounded carrier shows a combined entry and wrench cursor. Marked demolition targets cannot be boarded, so the worker action stays consistent with the job already assigned to that building.

“Eligible infantry board carriers and garrison buildings with left-click while selection and repairs remain distinct.”