Make UK paper analysis reproducible on Python 3.14

[MaxGhenis]

Summary

• add a checked-in UK paper reproduction script and README instructions
• let calculate_decile_impacts() reuse existing simulations and ensure internally when it constructs them
• fix UK and US output-model forward references and UK programme entity/output selection surfaced by the paper repro
• refresh uv.lock so the repo installs and the UK repro runs on Python 3.14

Verification

• uv run --python 3.14 --extra uk python examples/paper_repro_uk.py

Note

• uv run --python 3.14 pytest tests/test_intra_decile_impact.py currently fails before test collection because tests/conftest.py imports policyengine-us 1.390.0, and that package currently contains malformed YAML in policyengine_us/parameters/gov/states/ma/tax/income/credits/child_and_family/dependent_cap.yaml.
• This branch does not introduce that policyengine-us version; main currently resolves the same one in uv.lock.