Add test suite for plot functions

[Copilot]

No tests existed for any of the plot functions in pycrosstalker/plots/plot.py. This adds test/test_plots.py with 13 tests covering all public plot functions using the existing rawdata/ fixtures.

New tests (test/test_plots.py)

Test	Function	Notes
test_plot_cci / test_plot_cci_exp	plot_cci	CTR graph; EXP with log=True; validates returned fig/ax
test_plot_pca_LR_comparative_ggi / _no_ggi	plot_pca_LR_comparative	Both ggi=True and ggi=False code paths
test_plot_bar_rankings_cci / _cgi / _with_type_filter	plot_bar_rankings	cci mode, cgi mode, and type='LR' filter
test_plot_sankey / _with_target	plot_sankey	Without and with target gene filter
test_plot_volcane_fisher	plot_volcane	Fisher stats from data['stats']['EXP_x_CTR']
test_plot_clustermap	plot_clustermap	LR table pivot heatmap
test_plot_graph_clustermap / _exp_x_ctr	plot_graph_clustermap	CTR and differential EXP_x_CTR graphs

All tests use the Agg matplotlib backend for headless rendering and call plt.close("all") for cleanup.

Housekeeping

• Added __pycache__/ and *.pyc patterns to .gitignore to prevent build artifacts from being committed.

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