[BUG] Rust: default parallel cargo test SIGSEGVs on GPU tests; passes with --test-threads=1

[jamie8johnson]

Describe the bug

Running the Rust crate's test suite with cargo's default parallel test threads SIGSEGVs intermittently on GPU tests. Observed on unmodified main (78135be): ivf_flat tests crash under the default runner; the same tests pass reliably with --test-threads=1. We reproduced the identical pattern while developing the IVF-SQ (#2229) and refine (#2230) bindings — single-threaded always green, default runner intermittently fatal.

Steps/Code to reproduce bug

cd rust
cargo test -p cuvs              # intermittent SIGSEGV (ivf_flat and others)
cargo test -p cuvs -- --test-threads=1   # passes

Expected behavior

Either the tests are safe to run concurrently, or the harness enforces serialization so contributors don't chase phantom crashes.

Recommended fix

Near-term: enforce serial execution at the harness level rather than relying on contributor knowledge — either a .cargo/config.toml in rust/ setting [env] RUST_TEST_THREADS = "1", or the serial_test crate's #[serial] on GPU-touching tests; plus one line in the rust README. Root-cause follow-up: tests each create their own Resources, so the crash suggests shared global state in the C layer (stream pool, memory resource, or RMM current-device resource) isn't safe under concurrent Resources creation/destruction from multiple host threads — an audit of cuvsResourcesCreate/Destroy thread-safety would pin it down. We're happy to submit the harness-level guard as a small PR; the thread-safety audit likely needs maintainer eyes.

Environment details

Rust crate at main (78135be), conda libcuvs 26.06, CUDA 13.1, RTX 4000 (8GB), Rust 1.96.0.