Migrate primary residence value imputation to Populace

Context

policyengine-us-data PR PolicyEngine/policyengine-us-data#1085 added ACS-based primary_residence_value imputation from ACS VALP, fixing PolicyEngine/policyengine-us-data#1079.

That old PR is still open/draft in the superseded data repo, but the underlying question still matters for Populace: do we publish the PE-US-facing primary_residence_value input, and if so should it come from ACS VALP, SCF scf_primary_residence_value, or an explicitly documented bridge between the two?

This is related to the broader wealth/property backlog in #49, but narrower: make sure the primary-residence-value surface does not get lost inside the general asset-imputation audit.

Current Populace state

Local inspection finds scf_primary_residence_value declared in the US SCF wealth stage, but no obvious primary_residence_value output or ACS VALP source path.

Migration direction

Do not port the old policyengine-us-data staged-pipeline code directly. Translate the useful behavior into Populace source specs, export contracts, and diagnostics.

Questions to resolve:

• Whether primary_residence_value is a required PE-US input in the published populace-us artifact.
• Whether ACS VALP should provide the primary source for owner-occupied housing value, with SCF used for broader wealth decomposition, diagnostics, or fallback.
• Whether scf_primary_residence_value should remain separate, be bridged into primary_residence_value, or be formula/diagnostic-only.

Acceptance criteria

• Inventory/classify primary_residence_value and scf_primary_residence_value as populated, upstream-populated, formula-computed, diagnostic-only, or reviewed-excluded.
• If primary_residence_value should be published, add the source-stage spec and export mapping needed to populate it.
• Preserve the key behavioral guard from #1085: renters and non-owner/non-head records should not receive positive owner-occupied primary residence value.
• Add fixture/unit coverage for owner vs renter handling and household-head carry behavior.
• Add a release diagnostic or benchmark comparison against the chosen source/target, such as ACS owner-occupied home value totals/distribution or SCF housing wealth diagnostics.

Part of #49 and the broader migration audit in #46.

Start by reviewing the US SCF wealth stage declaration for scf_primary_residence_value and the prior policyengine-us-data PR #1085. Resolve the source and export contract, then add the relevant source-stage and fixture/unit coverage for owner, renter, and household-head cases. Done means the published artifact, diagnostics, and acceptance criteria consistently reflect the chosen mapping.