feat: Add condition_key support, root API exports, and plotting improvements

[aarmey]

Summary of changes

• Export bicv, prepare_dataset, plotting, and version at root scrise namespace
• Add condition_key parameter and adata alias to pf2() and bicv() for automatic condition index generation
• Generalize ThomsonNorm to control_pattern / control_conditions in plot_condition_factors
• Remove scanpy dependency in stability.py in favor of numpy sampling
• Guard against zero genes passing weight filter in plot_gene_factors
• Add unit tests for all new features and verify >85% test coverage