Implements Base26 encoding/decoding using uppercase A-Z as digits
(A=0, B=1, ..., Z=25). Input bytes are interpreted as a big-endian
unsigned integer and converted to/from radix 26.
Includes tests for the reference vector (EAT -> JYGBS), round-trips,
empty input, zero bytes, and binary data.
Closesgchq/CyberChef#1511
