Add IVF_RQ query bits and format reservations

[JingsongLi]

Summary

Adds IVF_RQ/RaBitQ support with a search-time query_bits tuning knob and reserves IVF_RQ v1 format fields for future multi-bit codes. Defaults remain conservative: query_bits=0 keeps the existing float-query path, and writers still emit 1-bit stored codes.

Changes

• Implement IVF_RQ/RaBitQ core indexing, serialization, and reader search paths.
• Add query_bits=0|4|8 APIs across Rust, C ABI, C++ wrapper, Java/JNI, and Python.
• Reserve IVF_RQ v1 header fields for num_bits, rotation_type, factor_layout, optional ex_codes, and optional error_factor.
• Keep current readers strict: future multi-bit or optional-section payloads are rejected until the scanner is implemented.
• Extend ANN benchmark knobs, storage golden fixtures, validation tests, and docs.

Testing

• cargo fmt --all
• cargo test --workspace -- --nocapture
• git diff --check

Notes

Current multi-bit storage is intentionally format-reserved only. Writers still produce num_bits=1 with no optional sections, so this PR does not enable multi-bit RQ by default.

[leaves12138]

Looks good to me. I reviewed the IVF_RQ query_bits API paths, format field validation, and language bindings; the Rust workspace tests, rustfmt check, and clippy all passed locally.