Add spec-driven ASEC support spine

[MaxGhenis]

Summary

• adds a packaged US support_spine.json declaring a period-relative current/prior ASEC pool
• adds shared support-spine manifest parsing and validation for declarative source roles, offsets, shares, and citations
• requires every support-spine source to declare an explicit share; the builder rejects extra --asec-h5 mappings in spec mode
• wires tools/build_us_puf_support_base.py so --support-spine-spec default resolves runtime YEAR=PATH ASEC inputs from the packaged spec
• keeps local paths out of the country package; the manifest owns roles and shares, while the build command supplies files
• lazily resolves the QRF fitter while preserving the existing puf_support.QRF test seam

Validation

• uv run --python 3.13 --package populace-build --extra us --group dev pytest packages/populace-build/tests/test_us_puf_support_base_builder.py packages/populace-build/tests/test_us_plan.py -q
• uv run --python 3.13 --package populace-build --extra us --group dev pytest packages/populace-build/tests/test_spec_only_country_packages.py packages/populace-build/tests/test_us_asec_pool.py packages/populace-build/tests/test_us_puf_support_base_builder.py packages/populace-build/tests/test_us_plan.py -q
• uv run --python 3.13 --package populace-build --extra us --group dev pytest packages/populace-build/tests/test_us_puf_support.py::test_puf_tax_detail_snaps_sparse_taxable_interest_to_observed_zero packages/populace-build/tests/test_us_puf_support.py::test_puf_tax_detail_imputation_reconciles_social_security_components packages/populace-build/tests/test_us_puf_support.py::test_puf_tax_detail_imputation_snaps_origination_years_to_donor_values packages/populace-build/tests/test_spec_only_country_packages.py packages/populace-build/tests/test_us_asec_pool.py packages/populace-build/tests/test_us_puf_support_base_builder.py packages/populace-build/tests/test_us_plan.py -q
• uv run --python 3.13 --package populace-build --extra us --group dev ruff check tools/build_us_puf_support_base.py packages/populace-build/tests/test_us_puf_support_base_builder.py packages/populace-build/tests/test_us_plan.py packages/populace-build/src/populace/build/source_manifest.py packages/populace-build/src/populace/build/us_runtime/__init__.py packages/populace-build/src/populace/build/us_runtime/puf_support.py
• uv run --python 3.13 --package populace-build --extra us --group dev ruff format --check tools/build_us_puf_support_base.py packages/populace-build/tests/test_us_puf_support_base_builder.py packages/populace-build/tests/test_us_plan.py packages/populace-build/src/populace/build/source_manifest.py packages/populace-build/src/populace/build/us_runtime/__init__.py packages/populace-build/src/populace/build/us_runtime/puf_support.py
• git diff --check
• bounded smoke build with --support-spine-spec default, 50 households per ASEC source, and --n-estimators 2; output summary confirmed current ASEC resolved to 2024, prior ASEC resolved to 2023, each at 0.5 share, with the packaged spec recorded in provenance

Review cycle

• Read-only review found two spec-contract gaps: shareless specs could fall back to equal shares, and spec mode ignored extra ASEC mappings.
• Both are fixed with negative tests.

Notes

This is a pipeline/spec refactor and smoke validation only. It does not publish or promote a new dataset.