Use incrementality module for ROAS computation in budget plots

[isofer]

From PR #2531 discussion with @juanitorduz.

The budget allocation plotting code (BudgetPlots.allocation_roas) computes ROAS directly via sample_response_distribution(). For consistency, we should route this through the incrementality module instead.

This could also open the door to computing ROAS at different time points within an allocation period (e.g., monthly within a quarterly budget), which would require updating the incrementality module to support arbitrary allocations and adding a roas_over_time plot.

Scope:

• Refactor ROAS computation in budget plots to use the incrementality module
• Explore support for time-varying ROAS within an allocation period