Track US geography and local-area source support

Source backlog

Copied from policyengine-us-data issues/PRs that remain relevant after the Populace rebuild:

• PolicyEngine/policyengine-us-data#1143: document district H5 boundary vintage.
• PolicyEngine/policyengine-us-data#1144, #1146: plan/audit congressional boundary transitions.
• PolicyEngine/policyengine-us-data#1145: canonical congressional district geojson provenance.
• PolicyEngine/policyengine-us-data#482: official Census metro areas for congressional districts / GEOADJ.
• PolicyEngine/policyengine-us-data#454: county-to-household mapping for ACA rating areas.
• PolicyEngine/policyengine-us-data#75: county as predictor for rent/property-tax ACS imputations.
• Related: #517, #470, #598, PR #1088.

Populace direction

Keep this in Populace's spec-driven source architecture. Geography vintages, source artifacts, and assignment constraints should be declared in manifests/config, while any generic geocoder/crosswalk operation stays in shared runtime code.

Acceptance criteria

• US source/release manifests expose boundary vintage for congressional district, county, CBSA/metro, and ACA rating-area geography used by a build.
• Local-area artifacts have provenance for district boundary vintage and canonical source files.
• County/rating-area and district mappings have fixture tests for known boundary-transition states.
• Geography predictors required for rent/property-tax/ACA assignment are declared in source specs and covered by tests.

Part of #46.

Start by locating Populace's spec-driven source manifests and the shared runtime code for source artifacts and geography operations. Trace how congressional district, county, CBSA/metro, and ACA rating-area inputs are declared, then inspect existing fixture tests. Done means manifests expose boundary vintages and provenance, required predictors are declared, and known transition mappings are covered by tests.