IVF index (src/indexes/ivf.rs):
- New IVF index: a centroid index plus a flat cluster scan dataset, with
   a CSR cluster layout.
- Enables IVFFlat, IVFPQ, IVF-HNSW, and multi-level IVF, with and
  without residuals.
- Add ivf_build / ivf_search binaries, run_experiments.py wiring, and
  SIFT / Dragon experiment configs.

Adapt to vectorium 0.4.0:
- Pin vectorium to tag v0.4.0.
- Follow the search-only  trait and the new  trait.