ixsurface 0.1.0

• Initial release.
• interaction_surface() for interactive 3D surface plots from fitted models.
• plot_crossings() for standalone crossing-point visualization.
• find_crossings() for programmatic crossing detection.
• interaction_surface_grid() for all-pairwise exploration.
• sim_factorial() for generating test data with known interactions.
• Support for lm, glm, gam, and any model with a predict() method.
• Automatic binning of continuous conditioning variables.
• Categorical axis support with tick label mapping.