Add unit tests for distance functions (L2, cosine, hamming)

Add test-only wrappers behind SQLITE_VEC_TEST compile flag to expose
static distance functions for unit testing. Includes tests for
distance_l2_sqr_float (4 cases), distance_cosine_float (3 cases),
and distance_hamming (4 cases). Print active SIMD flags at test start.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>