Add UK official geography crosswalk sources

[MaxGhenis]

Summary

• add public-source helpers for complete UK row-wise geography crosswalks without importing policyengine-uk-data
• update England/Wales OA rows to LAD23 and append Northern Ireland DZ2021 rows as OA-equivalent geography
• infer NI DZ -> Westminster constituency by active-postcode modal PCON, with source-vintage diagnostics and strict source validation
• export the source helpers from populace.build.uk and add regression coverage for missing/blank source joins, stale NI rows, and NI source set mismatches

Live validation

Using the incumbent GB OA crosswalk as input plus the public source helpers in this PR:

• constituencies: 650/650 covered and sampleable
• local authorities: 360/360 covered and sampleable
• NI rows: 3,780 DZ2021 rows
• NI constituency codes: 18
• postcode vintage gap: 214 unmatched active NI postcodes out of 49,192 (0.4350%), under the 1% guard

Checks

• uv run --frozen --package populace-build --group dev python -m pytest packages/populace-build/tests/test_uk_geography_sources.py packages/populace-build/tests/test_uk_rowwise_geography.py -q
• uv run --frozen ruff check .
• git diff --check
• uv run --frozen --all-packages --group dev python -m pytest -q
• env -u UV_FROZEN uv lock --check
• live source validation script against public ONS/NISRA sources and policyengine-uk-data target code lists

Review cycle

• First read-only review found stale NI filtering, lax E/W lookup coverage, incomplete NI source joins, and postcode source-vintage drift; fixed and retested.
• Second read-only review found blank LAD23 lookup values could leak through; fixed with explicit validation and a regression test.