Compute Fletcher Checksums (16, 32, and 64 Bit) // Computes Fletcher's position-dependent checksum in 16-, 32-, and 64-bit widths. Fletcher's checksum, devised by John G. Fletcher, provides error-detection properties approaching a cyclic redundancy check at lower computational cost. Input is processed as little-endian words with the final partial word zero-padded, so results are reproducible across platforms.