bug: IVF_HNSW_PQ recall assertions fail intermittently because k-means init is unseeded

test_create_ivf_hnsw_pq_4bit and test_create_ivf_hnsw_pq_multivec fail their
assert_ge!(recall, 0.5) check intermittently, on main as well as on PRs.

Observed in the last day:

Each run was otherwise green (e.g. 3146 passed, 1 failed), and the failing test
differs by runner, so this is not a regression from any one change.

The input is already deterministic: make_seeded_vector_batch
(rust/lance/src/index/vector/ivf/v2.rs:2615) pins Seed::from(42). The variance
comes from index training instead — rust/lance-index/src/vector/kmeans.rs:930
reads

// TODO: use seed for Rng.
let mut rng = SmallRng::from_os_rng();

so centroid initialization differs every run. With K = 10 the recall metric
moves in steps of 0.1 and the threshold is 5 of 10 neighbours, so a slightly
worse set of centroids flips the assertion.

Threading a seed through KMeansParams for the test path would make these
deterministic, which the existing TODO already anticipates. Lowering the
threshold would also stop the failures but would weaken the check.

Start with rust/lance-index/src/vector/kmeans.rs:930 and inspect how KMeansParams reaches training for test_create_ivf_hnsw_pq_4bit and test_create_ivf_hnsw_pq_multivec. Compare this with the seeded input setup in rust/lance/src/index/vector/ivf/v2.rs:2615, then run both named tests repeatedly. Done means centroid initialization is deterministic for these tests and the existing recall assertions pass without lowering their threshold.