[ny_hp_rates] Add fair default feasibility line analysis

[sherryzuo]

Summary

• Adds a new ny_hp_rates notebook for fair default feasible line analysis.
• Includes updated plots for fair-default comparisons and cross-subsidy lines.
• Adds the notebook to the ny_hp_rates Quarto render list.

Reviewer Focus

Please focus on whether the fair-default framing and cross-subsidy line plots are clear enough for review, and whether the notebook belongs in the project render list at this stage.

Non-Obvious Details

main was accidentally merged into directly first, then reverted without rewriting shared history. This branch has been rebuilt on top of the corrected main so the PR shows the intended two-file diff.

Test Plan

• Not run; PR only wires in and adds the analysis notebook.