Placement Validity Feedback Bounds and Validity POC

Adds the missing safety layer to placement:

• Press P to enter placement mode (ghost appears)
• Ghost is VALID only if:
  1. it is fully inside world bounds, and
  2. it does not overlap any existing placed object
• Ghost shows green when valid, red when invalid
• LMB commits only if valid (then exits placement mode)
• ESC or RMB cancels placement (exits placement mode)