Native exact flat-scan index over mirrored ids and vectors.
ETS remains the canonical record store for values and metadata. The native
resource keeps ids plus a contiguous vector matrix so an exact CPU scan is one
native SIMD call. :gpu, :gpu_min_size, and :gpu_fallback index options
enable a generation-aware resident matrix: warm searches upload only the
query, reduce top-k on the device, and read back compact hits.
Effective mutations invalidate the device snapshot. The next eligible query rebuilds it once; concurrent warm queries share the immutable matrix and use independent pooled scratch buffers. GPU top-k supports up to 64 returned results and follows the configured fallback policy above that limit.