Populate remaining US tax-input and reported-observation layers

[MaxGhenis]

Summary

Track the remaining US input-layer work as source/profile work in live Populace, independent of any incumbent comparison surface.

Inputs to populate or explicitly classify

Pure tax inputs that should be sourced from the PUF donor/impute stage:

• amt_foreign_tax_credit
• general_business_credit
• excess_withheld_payroll_tax
• prior_year_minimum_tax_credit
• early_withdrawal_penalty
• other_credits

Formula-owned output with missing upstream support:

• self_employed_health_insurance_ald should not be persisted directly. Populate the upstream self-employed health-premium inputs needed for the PolicyEngine-US formula.

Reported/comparison observations to classify:

• spm_unit_net_income_reported
• spm_unit_total_income_reported
• ssi_reported

These are not formula-consumed benefits/taxes; decide whether Populace should carry them as reported observations or document their absence as named exclusions.

Acceptance criteria

• The source/profile inventory records each variable as populated, upstream-populated, carried as reported observation, or reviewed-excluded.
• The writer still rejects formula-owned outputs as final inputs.
• Tests cover at least one populated PUF-derived credit, the SE-health upstream path, and the reported-observation classification.
• Release diagnostics expose any reviewed exclusions.

Supersedes live Populace issue #26, which framed this as an incumbent-comparison gap. Benchmark/incumbent comparison discussion now belongs in PolicyEngine/populace-benchmarks.

[MaxGhenis]

PE-US-data backlog migration note from #46: this issue should absorb remaining tax-input/reported-observation items from PolicyEngine/policyengine-us-data#1126, #1127 / PR #1128, #900, #253, #85, #619, #613, #1160, #227, #525, #526, #527, #529, #712, #219, #139, #138, #180, #193, and #185. The important constraint is to populate PE-US inputs or reported observations through Populace source specs; formula-owned outputs still must not be persisted as final inputs.