Budget optimizer does not account for pre-allocation-period carryover effects

[isofer]

From PR #2531 discussion with @juanitorduz.

The current budget optimizer doesn't take into account the carryover effect of budgets that were used before the allocation period. For example, when optimizing Q2 spend, the lingering effects of Q1 spend on Q2 outcomes are not considered.

This could lead to suboptimal allocation recommendations, particularly for channels with strong adstock/carryover effects.

Scope:

• Investigate how pre-period carryover should feed into the optimizer's objective function
• Update budget_allocator (or related code) to condition on prior-period spend