refactor(ivf): use turbo for distance calculation

[richyreachy]

Refactor IVF to use the Turbo quantizer APIs adopted by HNSW and Flat. Keep centroid training and preprocessing separate from posting encoding, and use per-query Turbo distance state for batched posting scans.

• Persist quantizer metadata and training state for reload.
• Preserve legacy index loading and converter fallbacks.
• Decode vectors for fetch and providers, including merges between legacy and Turbo indexes.
• Correct cosine centroid preprocessing and support batched centroid-query transforms.
• Reject incompatible column-major layouts and packed-only quantizers.