[codex] Add policyengine-uk validation and fix SCP parameters

[MaxGhenis]

Summary

Adds an actual validation path against policyengine-uk and fixes the Scottish Child Payment parameter drift that the legislative reference called out.

What changed

• added a real CI workflow that runs on pushes and PRs
• added a parity validator script that compares selected exact-overlap household scenarios against policyengine-uk
• added a small --scenario-json CLI path so the Rust engine can run deterministic single-household cases for validation
• corrected Scottish Child Payment weekly amounts for 2025/26 through 2029/30 to match the reference Python model path
• added a Rust regression test to lock those SCP year values in place
• updated the legislative reference appendix so it no longer lists the fixed SCP issues as outstanding

Why

The repo previously had internal tests and a broad OBR aggregate sanity check, but it did not have any automated validation against policyengine-uk, and GitHub Actions did not run validation on PRs at all. That left parameter drift undetected, including the incorrect Scottish Child Payment values already noted in the reference document.

Validation

• cargo test
• cargo build
• POLICYENGINE_UK_PATH=/Users/maxghenis/policyengine-uk python3 scripts/validate_against_policyengine_uk.py --rust-binary target/debug/policyengine-uk-rust

Notes

The parity suite intentionally covers exact-overlap scenarios only. It avoids known broader model-scope differences where the Rust implementation is still simplified relative to policyengine-uk (for example richer legacy-benefit and some historical-year behavior), so failures should be high-signal.