Add AVX2-optimized Hamming distance using VPSHUFB popcount

Implements distance_hamming_avx2() which processes 32 bytes per
iteration using the standard VPSHUFB nibble-lookup popcount pattern.
Dispatched when SQLITE_VEC_ENABLE_AVX is defined and input >= 32
bytes. Falls back to u64 scalar or u8 byte-at-a-time for smaller
inputs.

Also adds -mavx2 flag to Makefile for x86-64 targets alongside
existing -mavx.

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