Adds the missing “real world” rules to group movement:
- Box-select a group
- Drag any selected object to move the whole group (rigid formation)
- Movement is VALID only if:
- every selected object stays inside world bounds, and
- no selected object overlaps any non-selected object
- If invalid: movement is blocked (group stays at last valid position) and you get obvious red feedback
- Includes N to spawn test objects, C to clear, wheel zoom