Add UK raw-source and local-geography parity

[MaxGhenis]

Summary

This PR moves the UK replacement build closer to full Populace ownership by adding a raw-source parity contract and the long local-geography path needed to replace the legacy matrix workflow.

• Add a packaged UK source-stage manifest covering FRS, WAS, LCFS, ETB, NHS, SPI, FRS-only fill, capital gains, salary sacrifice, SLC student loans, national/local calibration, rail public-service calibration, road fuel/energy scaling, and row-wise OA/LA/constituency geography.
• Move UK stage order, donor citations, and structural-stage grouping out of Python lists and into source_stages.json; Python now derives UK_STAGE_NAMES, UK_DONORS, and UK_STRUCTURAL_SOURCE_STAGES from the manifest.
• Declare UK national and local calibration as Ledger-backed stages: read ledger_consumer_facts, compile uk_national_calibration / uk_local_geography target profiles, then calibrate weights against household metric tables.
• Add assigned long local-geography matrix/solver/runner support so local weights export as (area_type, area_code, household_id, weight) rows rather than preserving the dense area-by-household matrix artifact.
• Extend UK SPI support surfaces and tests for the SPI support channel and FRS-only fill variables.

Validation

• uv run ruff check .
• uv run --package populace-build --group dev python -m pytest packages/populace-build/tests/test_uk_source_manifest.py -q
• uv run --package populace-build --group dev python -m pytest packages/populace-build/tests -q
• python3 -m json.tool packages/populace-build/src/populace/build/uk/source_stages.json >/dev/null
• git diff --check
• Manifest source URL scan: all unique URLs returned HTTP 200, including https://github.com/PolicyEngine/arch-data
• Incumbent/eFRS reference scan over touched UK/docs/test surfaces: no matches
• Review cycle: independent read-only subagent review loop ended with no actionable findings remaining