Update the PolicyEngine bundle to 6.3.4 #3921

Problem

The Public API pins PolicyEngine .py 6.2.1, while policyengine.py 6.3.4 is published and the simulation API already serves it (PolicyEngine/policyengine-sim-api#797, deployed to production on 14 October 2026).

6.3.4 bundles:

• policyengine-core 3.33.0
• policyengine-us 2.2.1
• policyengine-uk 2.126.12
• spm-calculator 1.0.0

What 6.3.4 changes for this repository

• UK model 2.126.12 changes the installed catalog's counts. The installed-catalog test pins them exactly.
• US regions: state and congressional-district regions use the national dataset again (Temporarily filter US state and district regions from the national dataset policyengine.py#604), so every US region defaults to populace_us_2024.
• Poverty rates: Poverty.rate is None when nobody in a group is measured. The simulation API still reports such rates as 0, so the Public API's responses don't change.

Done when

• pyproject.toml, uv.lock and docker/Dockerfile pin policyengine[models]==6.3.4.
• The installed-catalog test passes on the 6.3.4 install.
• PR CI passes, including "Ensure PolicyEngine bundle is supported by simulation API".