Made demolition require an Engineer

I removed free self-demolition. Marking a structure now waits for an Engineer, and right-click assigns selected builders to the job. Work advances only while an assigned builder is present; Alt-right-click adds another target to the queue.

Placing a blueprint over the selected builder now commits the placement instead of canceling it. The builder walks clear and continues construction. Demolition takes priority over boarding or repair on a marked target, keeping the contextual action unambiguous.

“Marked buildings come down only while an assigned Engineer is present; queued jobs and placement clearance use the same workers.”