Add UK SPI support rows

[MaxGhenis]

Summary

• add a Populace-owned UK SPI support-row primitive matching the eFRS pattern: base FRS rows keep weight, SPI support rows are sampled FRS copies with remapped IDs and zero household weight
• preserve source_household_id/source_household_key/clone_index lineage so longwise local support counts do not treat the SPI copy as independent FRS support
• add channel-scoped source fill helper and export the eFRS stage-1 SPI income and stage-2 FRS-only fill variable surfaces

Tests

• uv run ruff check packages/populace-build/src/populace/build/uk/spi_support.py packages/populace-build/src/populace/build/uk/init.py packages/populace-build/tests/test_uk_spi_support.py
• uv run --package populace-build --group dev python -m pytest packages/populace-build/tests -q

Review

• Read-only subagent review found no actionable issues; added a regression for pre-existing rowwise lineage metadata based on residual-risk feedback.