High-performance scaling optimizations for large-scale datasets

[svdrecbd]

This PR introduces critical algorithmic and I/O optimizations to the kb_python core processing paths. By replacing sequential Python loops with vectorized operations and implementing memory-safe streaming, these changes enable the tool to scale gracefully to multi-million cell datasets.

Optimizations:

• collapse_anndata (Vectorized): Replaced manual sparse column loops with a single sparse matrix-matrix multiplication (X @ S).
  • Performance: Achieved a ~2900x speedup on a 100k sample and reduced full-scale (6.7M cells) processing time from ~1 hour to 3.4 seconds.
• do_sum_matrices (1-Pass Merge): Switched to a single-pass streaming merge with a temporary body file.
  • Performance: Improved runtime by 1.72x while maintaining O(1) memory safety.
  • Integrity: Uses strict integer arithmetic when processing count matrices to ensure bit-exact precision (addressing float rounding risks).
• generate_kite_fasta (KITE Ref Gen): Optimized collision detection from $O(N^2)$ to $O(N)$ using dictionary-based lookups.
  • Performance: Reduced runtime for 10,000 features from ~80s to ~3.5s.
• validate_mtx: Replaced full-file loading with header-only inspection using scipy.io.mminfo.
  • Performance: Reduced validation time by ~1800x, turning a linear file read into a constant-time check.

Dataset Used for Validation:
All performance metrics and parity checks were conducted using the following real-world dataset:

• Source: 10x Genomics PBMC 10k (Chromium v3)
• Matrix: Raw feature-barcode matrix (contains all 6.7M+ detected barcodes)
• Dimensions: 6,794,880 barcodes x 33,538 features
• URL: 10x PBMC 10k v3 Raw Dataset
  (https://cf.10xgenomics.com/samples/cell-exp/3.0.0/pbmc_10k_v3/pbmc_10k_v3_raw_feature_bc_matrix.tar.gz)

Scientific Parity & Compliance:

• Numerical Parity: Confirmed exact scientific parity ((X_old != X_new).nnz == 0) across all optimized paths using the 6.7M cell dataset.
• MatrixMarket Compliance: Body lines are explicitly formatted as integers when multimapping is disabled, ensuring strict compliance with the .mtx format.
• Regression Suite: All existing tests pass. Added new regression tests for do_sum_matrices covering integer formatting and complex sparse overlapping merges.