Function pick_quantization 

pub fn pick_quantization(
    candidate_set_size: usize,
    target_recall: f32,
) -> QuantizationKind

Pick the best quantization tier for a given candidate set size and target recall requirement.

Heuristic

• Small sets (< 1 000): FP32 — the set fits in cache; no compression gain.
• Medium sets (1 000 – 99 999):
  • target recall ≥ 0.97 → SQ8 (high fidelity, 4× compression)
  • otherwise → PQ (8× compression, slightly lower recall)
• Large sets (≥ 100 000):
  • target recall ≥ 0.95 → BBQ (centroid-centered 1-bit + rerank)
  • otherwise → RaBitQ (raw 1-bit, fastest scan)