feat: implement flow parameter support in comparison plotters (fixes #594)

[andrzejnovak]

Add comprehensive support for flow parameter in comparison plotting functions including plot_data_model_comparison(), plot_two_hist_comparison(), and plot_model(). Key improvements:

• Preserve original histogram flow information through plotting pipeline
• Pass original histograms to histplot() for proper flow bin detection
• Prevent xlim reset when flow="show" to preserve extended axis range
• Filter 'flow' parameter before passing to funcplot() compatibility
• Add flow-aware ratio computation in comparison panels

Supports all flow options: "hint" (diamond markers), "show" (extended range), "sum" (included in edge bins), and "none".

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]

[andrzejnovak]

@copilot maybe you can fix this to stop breaking the other tests

[Copilot]

@andrzejnovak I've opened a new pull request, #659, to work on those changes. Once the pull request is ready, I'll request review from you.