refactor: integrate configurable RaBitQ quantization into standard HNSW

[richyreachy]

Summary

Add RaBitQ as a quantizer option for the standard HNSW index, using the existing original-vector provider for graph construction. This allows HNSW to build from original FP32 vectors while storing RaBitQ-encoded vectors.

• Support configurable 1–8 bit encoding, defaulting to 7 bits, for L2, inner product, and cosine.
• Persist quantizer configuration and rotation state for save/reopen.
• Require an original-vector provider for insertion; reopened indexes can serve queries without one.
• Add an optional-OpenMP compatibility patch and document configuration and usage.

The existing standalone HNSW-RaBitQ index remains unchanged. This implementation uses zero-centered, training-free quantization and scalar distance kernels.