Implement network scoped broadcasts using user-selected IP

[tintinhamans]

Currently, broadcasts use 255.255.255.255 regardless of user IP selection. Update the network code so broadcasts are sent using the subnet's broadcast address for the user-selected IP/interface from the options menu. This will improve LAN discovery and reliability when multiple network adapters are present.

Tasks:

• Send broadcasts to the subnet broadcast address for the selected interface, not 255.255.255.255.
• Ensure incoming broadcast packets are only received on the user-selected interface/subnet (bind sockets to the chosen IP).
• Gate this new behavior behind RETAIL_COMPATIBLE_NETWORKING to preserve legacy compatibility.
• Tie broadcast addressing and receiving logic to the chosen network interface in options.
• Update initialization and sending/receiving logic accordingly.

Example:

• Selected IP: 192.168.0.50, Mask: 255.255.255.0
• Broadcast = (IP & Mask) | ~Mask = 192.168.0.255