fix(ext/node): allow IPv6 multicast addresses in UDP addMembership/dropMembership

[kajukitli]

The validation in addMembership and dropMembership was only checking for IPv4 addresses using isValidIPv4Address(), making the IPv6 code path unreachable:

addMembership(multicastAddress: string, interfaceAddress?: string): number {
  if (!isValidIPv4Address(multicastAddress, interfaceAddress)) {
    return codeMap.get("EINVAL")!;  // Always fails for IPv6!
  }
  // ...
  if (this.#family === "IPv6") {
    op_node_udp_join_multi_v6(this.#rid, multicastAddress, 0);  // Unreachable
  }
}

This refactors validation to check addresses based on the socket family:

• IPv4 sockets validate IPv4 multicast + optional interface addresses
• IPv6 sockets validate IPv6 multicast addresses (interface is by index, not address)

Also uses the existing isIP() helper instead of manual octet parsing.