feat: add HNSW RaBitQ with rotated storage and staged search

[CLiqing]

issue: #1747

Summary

Add HNSW_RABITQ to Knowhere with L2, inner product and cosine support.

• Build the graph using FP32 distances, then populate random-rotated Faiss RaBitQ storage using bounded encoding batches.
• Support database precision rbq_bits=1..9 (default 1) and request-local rbq_bits_query=0..8 (default 4) for search, range search and iteration.
• Use a one-bit estimate followed by multi-bit scoring during graph traversal, with metric-aware distance adapters and optimized query-estimate/full-distance SIMD kernels. Batch four independent qb=4 estimates with query-plane reuse and prefetch.
• Preserve original input data for cosine norm correction and optional refinement; support existing refine codecs and refine_k.
• Cover filtering/brute-force fallback, serialization, distance-by-ID and index registration. Keep the common HNSW input conversion unchanged; RaBitQ-specific bounded encoding lives in the bundled Faiss contrib utility.

Faiss relationship and scope

This selectively adapts the standard RaBitQ layout and staged-search approach from facebookresearch/faiss#5526, reference d8a85956060c7567ff40dc1eb0a77578533d8bad, to Knowhere's bundled Faiss. FP32 graph construction, rotation, cosine, filtering, refinement and request-local query parameters are Knowhere integration responsibilities.

New index/adaptor classes are under faiss/cppcontrib/knowhere. Existing core RaBitQ changes add a batched estimate interface and optimize SIMD kernels and unaligned-load safety.

Current boundaries

• Immutable after initial build; mmap, multi-vector and embedding-list support are not advertised.
• The parent Knowhere index must outlive its iterators, consistent with the current common iterator ownership limitation.
• ARM64, early AMD and native Sapphire Rapids hardware execution remain CI/platform follow-ups; local dispatch overrides and SPR syntax checks do not replace hardware coverage.

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/kind feature