Fix Stage 12 UK local-authority boundary metadata

[anth-volk]

Fixes #704

Summary

• package common UK local-authority names plus separate LAD22 and LAD23 rendering coordinates in the Stage 12 executor image
• detect the boundary version from the complete dataset's la_code_oa values before regional scoping, with explicit failures for mixed, unsupported, or unidentifiable configurations
• carry the detected boundary version through typed Parquet metadata for both child simulations and require agreement before report aggregation
• enrich Stage 12 local-authority results from the detected boundary version while preserving the existing GCS lookup for non-Stage-12 calculations
• remove the obsolete Stage 12 requirement for local_authorities_2021.csv and document the temporary ownership and removal path

This supersedes PolicyEngine/policyengine-uk-data#489 and PolicyEngine/policyengine.py#542. It does not add an API route, database schema, environment variable, or dataset-name routing rule.

Verification

• make test
• executor Ruff lint and format checks
• Pyright on every changed executor and contract source module
• contract, metadata, artifact, coordinator, aggregation, output, worker-validation, and entry-infrastructure focused tests
• built the contract wheel and verified that all three CSV resources are included
• built both linux/amd64 service images from their repository Dockerfiles

The first executor image attempt encountered a transient PyPI TLS disconnect while downloading plotly; the identical retry completed successfully.