Add UK local candidate runner

[MaxGhenis]

Summary

• add a Populace-owned UK local candidate runner for explicit area/target tables, country-specific metric generation, stacked solving, and longwise local geography export
• add optional populace-build[uk] engine extra while keeping PolicyEngine-UK imports lazy and avoiding policyengine_uk_data
• extend local support diagnostics to include zero-support areas, cloned source-household support, and effective sample size

Review fixes

• validates PolicyEngine simulation household order before reindexing metrics to the sorted household frame
• adds regression coverage for reversed simulation household order
• adds regression coverage for zero-support areas and cloned source-household support

Verification

• uv run --frozen --package populace-build --group dev python -m pytest packages/populace-build/tests/test_uk_local_runner.py packages/populace-build/tests/test_uk_local_geography.py packages/populace-build/tests/test_uk_local_targets.py packages/populace-build/tests/test_uk_local_solver.py -q
• uv run --frozen --all-packages --group dev python -m pytest -q
• uv run --frozen ruff check .
• env -u UV_FROZEN uv lock --check
• git diff --check

Notes

This is the reusable candidate-build path for the future UK variant="local"; the real PolicyEngine-UK H5 integration/pilot artifact build remains the next production validation step.