Hide IVF behind SQLITE_VEC_EXPERIMENTAL_IVF_ENABLE, default off

Rename SQLITE_VEC_ENABLE_IVF to SQLITE_VEC_EXPERIMENTAL_IVF_ENABLE and
flip the default from 1 to 0. IVF tests are automatically skipped when
the build flag is not set.

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