Bump policyengine to 0.12.1 (intra-decile double-counting fix) #410

Context

policyengine 0.12.0 has a bug where intra_decile_impact and intra_wealth_decile_impact double-count household percentage income changes, causing incorrect bucket classification in the economy comparison output.

This was fixed in PolicyEngine/policyengine.py#239 and will be released as 0.12.1.

Action

Update the pinned policyengine dependency in projects/policyengine-api-simulation/pyproject.toml from 0.12.0 to 0.12.1.