Build UK geography crosswalk from official sources

[MaxGhenis]

Summary

• add official-source E/W and Scotland loaders/builders so Populace can compose the complete UK row-wise geography crosswalk from ONS, NRS, NISRA, and postcode sources
• keep the existing base-crosswalk repair function for compatibility, but add build_official_uk_geography_crosswalk() as the source-independent path
• harden source validation, ZIP CSV selection, transient source-download retries, and NI active-postcode handling
• export the new public UK geography helpers and cover them with synthetic source tests

Live validation

• official crosswalk build: 239,023 rows
• country rows: England 178,605; Wales 10,275; Scotland 46,363; Northern Ireland 3,780
• country populations: England 56,490,284; Wales 3,107,463; Scotland 5,440,284; Northern Ireland 1,903,168
• target coverage using PE uk-data target lists for validation only: 650/650 constituencies covered and sampleable; 360/360 local authorities covered and sampleable
• one extra official LA code appears outside the PE target list: N09000011 (Northern Ireland), with no missing PE target LAs

Row-wise dataset pilot

Using /Users/maxghenis/.claude-worktrees/populace-uk-build/artifacts/populace_uk_2023.h5 and the official crosswalk:

• n=1 clone: person (1,157,100, 104), benunit (618,980, 12), household (535,080, 68), exact weight preservation, zero missing geo rows, 650 constituencies assigned, no duplicate source-household/constituency pairs
• n=2 clone: person (2,314,200, 104), benunit (1,237,960, 12), household (1,070,160, 68), exact weight preservation, zero missing geo rows, 650 constituencies assigned, no duplicate source-household/constituency pairs

Checks

• uv run pytest packages/populace-build/tests/test_uk_geography_sources.py packages/populace-build/tests/test_uk_rowwise_geography.py packages/populace-build/tests/test_uk_rowwise_dataset.py -q
• uv run ruff check packages/populace-build/src/populace/build/uk/geography_sources.py packages/populace-build/src/populace/build/uk/__init__.py packages/populace-build/tests/test_uk_geography_sources.py
• uv run ruff format --check packages/populace-build/src/populace/build/uk/geography_sources.py packages/populace-build/src/populace/build/uk/__init__.py packages/populace-build/tests/test_uk_geography_sources.py
• git diff --check origin/main...HEAD
• env -u UV_FROZEN uv lock --check
• uv run --all-packages pytest -q

Review

• /cycle read-only review pass 1 found the NI blank-string doterm active-postcode issue; fixed with regression coverage
• /cycle read-only review pass 2 found no remaining actionable findings