Hex Address to Dotted IPv4

Network tools often print a 32-bit address as hexadecimal text. Implement hex_to_ipv4, which turns that text into dotted-decimal IPv4 notation, or returns the string INVALID when the text is not in the accepted format.

The accepted format is exactly eight hexadecimal digits (0-9, a-f, A-F, ASCII only), optionally preceded by 0x or 0X. Anything else is INVALID: surrounding whitespace, signs, separators, other digits and any other length. Every byte of the address is written as two digits and the leftmost byte is the first octet. All 32-bit patterns are valid, including those whose first bit is set. The output has four decimal octets joined by dots with no leading zeros. Each test is a single call.