Function hamming_distance 

pub fn hamming_distance(a: &[u8], b: &[u8]) -> u32

Compute Hamming distance between two binary vectors

SIMD-optimized using:

• AVX2: _mm256_xor_si256 + manual popcnt
• AVX-512: _mm512_popcnt_epi64 (if available)
• NEON: veorq_u8 + vcntq_u8

Falls back to scalar popcnt if no SIMD available.