upstream: contribute IVF-SQ Rust bindings to rapidsai/cuvs

[jamie8johnson]

What

Contribute Rust bindings for the IVF-SQ (scalar quantization) index to rapidsai/cuvs, mirroring the wrapper pattern from our v26.06.00 contributions ("feat(rust): add serialize/deserialize support for CAGRA index", "feat(rust): add search_with_filter to CAGRA Index").

Why

cuVS v26.06.00 shipped IVF-SQ as C++ and C APIs only ("IVF-SQ C++ API", "IVF-SQ C API") — no Rust surface. Scalar quantization stores vectors at ~4x smaller than f32. For cqs that matters as slots multiply: every A/B slot carries a full per-model vector index, and multi-slot experiments are the standard retune workflow.

Scope (upstream, in cuvs Rust crate)

• Wrap the IVF-SQ C API: index build params, build, search, serialize/deserialize
• Follow the existing cagra module structure in the Rust crate (we know this surface — we wrote parts of it)
• Tests mirroring the CAGRA Rust test layout

cqs-side follow-up (separate, after upstream merge + release)

• Optional IndexBackend impl behind cuda-index, selectable for memory-constrained slots
• A/B recall-vs-memory on the v3.v2 fixture before any default change

Prereqs

• We must be on official cuvs 26.6 first (fork retirement in progress)
• Conda libcuvs and crate pin move in lockstep (RMM → CCCL MR migration in 26.06 makes ABI matching non-negotiable)

[jamie8johnson]

Bindings implemented, reviewed, and GPU-tested — ready for upstream submission.

• Branch: rust-ivf-sq (commit 2beefce7) in ~/cuvs-contrib (clone of rapidsai/cuvs, branched off main — RAPIDS retired branch-XX.YY)
• Full C API wrapped (17/18 symbols; GetCenters deliberately skipped matching cagra/ivf_pq, commented in code)
• 9/9 GPU tests pass; validated against both the v26.06.00 tag and current main (header byte-identical)
• Code review verdict: ready-to-push (FFI-safe, conventions match siblings, bindings.rs additive)
• Submission is two commands, documented in ~/cuvs-contrib/UPSTREAM_PR.md (push to fork + gh pr create); PR body prepared in UPSTREAM_PR_BODY.md
• Monthly follow-up routine armed: cuvs-ivfsq-pr-follow (cloud, follow-only, first run Jul 3)

cqs-side integration (IndexBackend + patch override + recall/memory A/B) starts once the upstream PR exists, per this issue's plan.

[jamie8johnson]

Upstream PR submitted: NVIDIA/cuvs#2229 (branch rust-ivf-sq, fork pushed). Monthly follow-only routine cuvs-ivfsq-pr-follow now has a live target — first scheduled run Jul 3. cqs-side IndexBackend integration can start any time against the fork branch via [patch.crates-io].