Improves robustness, compatibility, and documentation across the pathways workflow.

Highlights

• Added support for ecoinvent 3.12 LCIA method mapping.
• Improved handling of headered matrix/index CSV files (A_matrix.csv, B_matrix.csv, A_matrix_index.csv, B_matrix_index.csv).
• Fixed export behavior so export_results() writes parquet output without a pandas index column.
• Strengthened datapackage validation and error reporting.
• Reduced startup side effects with lazy imports in pathways.init.
• Narrowed broad exception handling in key execution paths.
• Added optional control for postprocessing parallelism via postprocess_multiprocessing.
• Suppressed known third-party deprecation warnings during tests for cleaner CI output.
• Updated README and Sphinx docs to reflect current API and behavior.

Quality and Testing

• Added/updated tests for:
  • ecoinvent version resolution (including 3.12),
  • datapackage validation behavior,
  • parquet export index handling.