Implement visualization methods for Model class

[jgallowa07]

Re-implement the four key visualization methods from v1.x Model class for the v2.0 jaxmodels backend.

Methods Needed

1. plot_epistasis(condition, ax) - Plot global epistasis transformation curve
2. plot_pred_accuracy(condition, ax) - Scatter plot of observed vs predicted
3. plot_param_hist(parameter, ax) - Histogram of parameter distribution
4. mut_param_heatmap(parameter, conditions) - Interactive Altair heatmap

Tasks

• Implement plot_epistasis() using jaxmodels GE function
• Implement plot_pred_accuracy() using jaxmodels predictions
• Implement plot_param_hist() accessing jaxmodels parameters
• Implement mut_param_heatmap() with Altair
• Write tests for each visualization method
• Add doctest examples

Acceptance Criteria

• All four methods produce correct plots
• Plots accurately reflect jaxmodels parameters/predictions
• Tests verify plot generation succeeds
• Doctests demonstrate usage