feat: add high-performance HNSW RaBitQ index

[CLiqing]

What this PR does

Adds an end-to-end HNSW_RABITQ index to Knowhere together with the embedded Faiss RaBitQ kernels required for competitive graph-search performance.

• exposes 1-9 bit database codes for FP32/FP16/BF16 input
• supports L2 and inner product search
• builds the graph with exact FP32 storage, then attaches immutable rotated RaBitQ storage
• supports request-local query quantization for the 1-bit path and optional refinement
• adds a dense-packed multi-bit row layout for HNSW
• adds single and batch-4 scalar/AVX2/AVX-512/Sapphire Rapids scorers
• keeps BMI2 operations behind a runtime capability gate
• supports search, range search, iterator, distance-by-id, and serialization round trips

Scope and limitations

This PR is intentionally scoped to HNSW. The dense-packed layout is selected by HNSW_RABITQ; DiskANN and IVF integration are not included.

The initial implementation does not support cosine, scalar partition builds, mmap, incremental add after construction, or direct graph construction from RaBitQ code-to-code distances.

Validation

• Knowhere [hnsw_rabitq]: 6 test cases, 377,568 assertions passed
• Faiss RaBitQ*: 16 SIMD correctness tests passed
• Release knowhere_tests target built successfully on the latest main (a337f6a7)