Remove utils/kmeans.rs and migrate disk partitioning to diskann-quantization KMeans

[Copilot]

diskann-providers/src/utils/kmeans.rs was the last home-grown KMeans implementation remaining after PQ moved to diskann-quantization. This removes it entirely, replacing its users with the quantization crate's kmeans_plusplus_into + lloyds API.

Changes

• Deleted diskann-providers/src/utils/kmeans.rs — removes k_means_clustering, k_meanspp_selecting_pivots, run_lloyds, and all helpers (~989 lines).
• Removed k_means_clustering / k_meanspp_selecting_pivots / run_lloyds from diskann-providers::utils public API.
• diskann-disk/src/utils/partition.rs — the only live user; now calls kmeans_plusplus_into + lloyds via MatrixBase views:

  let data_view = MatrixBase::try_from(train_data_float.as_slice(), num_train, train_dim)?;
  let mut centers = MatrixBase::try_from(pivot_data.as_mut_slice(), num_parts, train_dim)?;
  kmeans_plusplus_into(centers.as_mut_view(), data_view, rng)
      .map_err(|e| ANNError::log_pq_error(e.to_string()))?;
  lloyds(data_view, centers.as_mut_view(), MAX_K_MEANS_REPS);

• pq_construction.rs — removed the already-dead generate_optimized_pq_pivots / opq_quantize_all_chunks / copy_chunk_centroids_to_full_table functions (and their test) that were the only remaining callers of the old API within this file.
• Benchmarks (kmeans_bench.rs, kmeans_bench_iai.rs) — updated to benchmark the diskann-quantization implementation.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.27%. Comparing base (de98ea6) to head (996064c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-disk/src/utils/partition.rs	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #920      +/-   ##
==========================================
- Coverage   89.34%   89.27%   -0.07%     
==========================================
  Files         444      443       -1     
  Lines       83986    83131     -855     
==========================================
- Hits        75036    74217     -819     
+ Misses       8950     8914      -36     

Flag	Coverage Δ
miri	89.27% <63.63%> (-0.07%)	⬇️
unittests	89.11% <63.63%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-providers/src/model/pq/pq_construction.rs	91.03% <ø> (-1.12%)	⬇️
diskann-disk/src/utils/partition.rs	91.91% <63.63%> (-0.64%)	⬇️

... and 2 files with indirect coverage changes

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