Verify US wealth, property, and asset imputation backlog

[MaxGhenis]

Source backlog

Copied from policyengine-us-data issues/PRs that still matter to Populace's US input surface:

• Impute primary_residence_value from ACS VALP into CPS outputs policyengine-us-data#1079, #1080, #1081 / PR #1085: ACS primary residence value imputation and publication.
• #230: home equity from SCF.
• #267, #281: vehicle ownership / auto-loan eligibility inputs.
• #835: net-worth aggregate decomposition.
• #734: replace SCF net-worth target with Fed Financial Accounts net worth.
• Related SPM/source-gap items may overlap with Track SPM-specific input gaps: housing, WIC, school meals, child support, workers comp, and expense deductions #32.

Populace direction

Populace already declares SCF wealth stages and scf_primary_residence_value; this issue is to verify the PE-US-facing variable surface and target/diagnostic coverage are complete.

Acceptance criteria

• Inventory each wealth/property/asset variable as populated, upstream-populated, formula-computed, diagnostic-only, or reviewed-excluded.
• Confirm whether primary_residence_value itself, home equity, vehicles, auto-loan variables, and net-worth components are present in the published PE-US input surface when needed.
• Add source-stage specs and tests for any missing source-backed inputs.
• Use Fed Financial Accounts / other external benchmarks as diagnostics or hard targets only after source role is explicitly classified.

Part of #46.