Buildings need physics colliders (spray paint hits, block cars/player)

[nigel-hall-codes]

Auto-captured and driven end-to-end by /sdlc capture auto.

Buildings currently have no collision: cars and the on-foot player pass straight through them, and spray-paint raycasts do not register hits on building surfaces. Add colliders to building geometry so that:

• The graffiti spray raycast lands on building walls.
• Cars collide with buildings instead of driving through.
• The on-foot player collides with buildings instead of walking through.

No human gating on capture, work, or merge.