Fix alignment UB in distance_hamming_u64

Casting unaligned blob pointers to u64* is undefined behavior on
strict-alignment architectures. Use memcpy to safely load u64 values
from potentially unaligned memory (compilers optimize this to native
loads on architectures that support unaligned access).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>