perf(index): parallelize kmeans centroid recomputation - #8560

Summary

• recompute KMeans centroids by partitioning input vectors across workers so each input vector is scanned once
• merge thread-local centroid accumulators in parallel and cap their aggregate allocation at 64 MiB
• add correctness coverage for f16, f32, f64, ignored memberships, empty-cluster splitting, and the memory cap
• add a focused Criterion benchmark and include it in the existing Rust benchmark workflow

The previous implementation divided centroids among workers, but every worker scanned the complete input. That made centroid recomputation perform O(num_vectors * workers) input reads. This changes the input scan to O(num_vectors) while avoiding shared-write contention.

Closes #6369.

Validation

Passed locally:

• cargo fmt --all -- --check
• cargo check -p lance-index --tests --benches --profile bench -j 1
• cargo clippy -p lance-index --tests --benches --profile bench -j 1 -- -D warnings
• git diff --check

The optimized test/benchmark linker exceeded the available resources on the local 7.6 GiB development machine, so executable test and benchmark runs are intentionally delegated to GitHub Actions. The PR is draft pending those results.