Add UK rowwise dataset build driver

[MaxGhenis]

Summary

• add tools/build_uk_rowwise_dataset.py to build a row-wise UK local-geography H5 from a compact Populace UK H5
• support generated or supplied official geography crosswalks, optional constituency/LA coverage checks, manifest hashes, weight-preservation diagnostics, and local-support summaries
• document the local build command in packages/populace-build/README.md

Real build smoke

Using the current compact UK 2023 H5, supplied official crosswalk, and --n-clones 2, the driver produced a 2.62 GB row-wise H5 with:

• person rows: 2,314,200; benunit rows: 1,237,960; household rows: 1,070,160
• household weight delta: 0.0
• missing geography rows: 0
• constituency coverage: 650/650; local-authority coverage: 360/360 PE target codes
• assigned geographies: 650 constituencies and 361 local authorities including the extra NI LA in official source data
• duplicate source-household/constituency pairs: 0
• weakest support: 551 household rows by constituency; 35 by local authority

Checks

• uv run ruff check tools/build_uk_rowwise_dataset.py packages/populace-build/tests/test_uk_rowwise_build_driver.py
• uv run ruff format --check tools/build_uk_rowwise_dataset.py packages/populace-build/tests/test_uk_rowwise_build_driver.py
• git diff --check
• env -u UV_FROZEN uv lock --check
• uv run --project packages/populace-build --extra uk python -m pytest packages/populace-build/tests/test_uk_rowwise_build_driver.py -q
• uv run --project packages/populace-build --extra uk python -m pytest packages/populace-build/tests/test_uk_rowwise_build_driver.py packages/populace-build/tests/test_uk_rowwise_dataset.py packages/populace-build/tests/test_uk_rowwise_geography.py -q
• uv run --all-packages python -m pytest -q

Review cycle: final read-only review reported no actionable findings.