Add tier-separated TOB revenue variables

Adds OASDI and Medicare HI specific trust fund revenue variables:
- tob_revenue_oasdi: Tier 1 (0-50%) revenue
- tob_revenue_medicare_hi: Tier 2 (50-85%) revenue

Uses proportional allocation of total TOB revenue.

Includes tier 1 and tier 2 taxable SS variables from PR PolicyEngine#6747.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: MaxGhenis

policyengine_us/tests/policy/baseline/gov/ssa/revenue/test_tob_with_lsr.py

@@ -0,0 +1,40 @@
+"""
+Test TOB revenue variable with labor supply responses.
+"""
+import pytest
+from policyengine_us import Microsimulation
+from policyengine_core.reforms import Reform
+
+
+def test_tob_revenue_baseline():
+    """TOB revenue should be positive in baseline."""
+    sim = Microsimulation()
+    tob = sim.calculate("tob_revenue_total", period=2026)
+    assert tob.sum() > 0
+
+
+def test_tob_revenue_with_lsr():
+    """TOB revenue should work with labor supply responses."""
+    lsr_params = {
+        "gov.simulation.labor_supply_responses.elasticities.income": {
+            "2024-01-01.2100-12-31": -0.05
+        }
+    }
+    reform = Reform.from_dict(lsr_params, country_id="us")
+    sim = Microsimulation(reform=reform)
+
+    # Should not raise RecursionError
+    tob = sim.calculate("tob_revenue_total", period=2026)
+    income_tax = sim.calculate("income_tax", period=2026)
+
+    assert tob.sum() > 0
+    assert income_tax.sum() > 0
+
+
+if __name__ == "__main__":
+    print("Testing TOB revenue...")
+    test_tob_revenue_baseline()
+    print("✓ Baseline works")
+    test_tob_revenue_with_lsr()
+    print("✓ LSR works")
+    print("\n✅ All tests passed!")

policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/irs_gross_income/social_security/taxable_social_security_tier_1.py

@@ -0,0 +1,58 @@
+from policyengine_us.model_api import *
+
+
+class taxable_social_security_tier_1(Variable):
+    value_type = float
+    entity = TaxUnit
+    definition_period = YEAR
+    label = "Taxable Social Security (tier 1)"
+    documentation = "Taxable Social Security from 0-50% taxation tier, credited to OASDI trust funds"
+    unit = USD
+    reference = "https://www.law.cornell.edu/uscode/text/26/86#a_1"
+
+    def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.irs.social_security.taxability
+        gross_ss = tax_unit("tax_unit_social_security", period)
+        combined_income = tax_unit(
+            "tax_unit_combined_income_for_social_security_taxability", period
+        )
+        filing_status = tax_unit("filing_status", period)
+        status = filing_status.possible_values
+        separate = filing_status == status.SEPARATE
+        cohabitating = tax_unit("cohabitating_spouses", period)
+
+        base_amount = where(
+            separate & cohabitating,
+            p.threshold.base.separate_cohabitating,
+            p.threshold.base.main[filing_status],
+        )
+        adjusted_base_amount = where(
+            separate & cohabitating,
+            p.threshold.adjusted_base.separate_cohabitating,
+            p.threshold.adjusted_base.main[filing_status],
+        )
+
+        under_first_threshold = combined_income < base_amount
+        under_second_threshold = combined_income < adjusted_base_amount
+
+        combined_income_excess = tax_unit(
+            "tax_unit_ss_combined_income_excess", period
+        )
+
+        # Tier 1 amount (IRC §86(a)(1))
+        amount_under_paragraph_1 = min_(
+            p.rate.base.benefit_cap * gross_ss,
+            p.rate.base.excess * combined_income_excess,
+        )
+
+        # Bracket amount when in tier 2 (IRC §86(a)(2)(A)(ii))
+        bracket_amount = min_(
+            amount_under_paragraph_1,
+            p.rate.additional.bracket * (adjusted_base_amount - base_amount),
+        )
+
+        return select(
+            [under_first_threshold, under_second_threshold],
+            [0, amount_under_paragraph_1],
+            default=bracket_amount,
+        )

policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/irs_gross_income/social_security/taxable_social_security_tier_2.py

@@ -0,0 +1,16 @@
+from policyengine_us.model_api import *
+
+
+class taxable_social_security_tier_2(Variable):
+    value_type = float
+    entity = TaxUnit
+    definition_period = YEAR
+    label = "Taxable Social Security (tier 2)"
+    documentation = "Taxable Social Security from 50-85% taxation tier, credited to Medicare HI trust fund"
+    unit = USD
+    reference = "https://www.law.cornell.edu/uscode/text/26/86#a_2"
+
+    def formula(tax_unit, period, parameters):
+        total_taxable = tax_unit("tax_unit_taxable_social_security", period)
+        tier_1 = tax_unit("taxable_social_security_tier_1", period)
+        return total_taxable - tier_1

policyengine_us/variables/gov/ssa/revenue/tob_revenue_medicare_hi.py

@@ -0,0 +1,31 @@
+from policyengine_us.model_api import *
+
+
+class tob_revenue_medicare_hi(Variable):
+    value_type = float
+    entity = TaxUnit
+    definition_period = YEAR
+    label = "Medicare HI trust fund revenue from SS benefit taxation (tier 2)"
+    documentation = "Tax revenue from tier 2 (50-85%) Social Security benefit taxation credited to Medicare HI trust fund"
+    unit = USD
+
+    def formula(tax_unit, period, parameters):
+        """
+        Calculate Medicare HI trust fund revenue from tier 2 SS taxation.
+
+        Allocates total TOB revenue to Medicare HI based on tier 2's proportion
+        of total taxable SS.
+        """
+        # Get total TOB revenue
+        total_tob = tax_unit("tob_revenue_total", period)
+
+        # Get tier amounts
+        tier1 = tax_unit("taxable_social_security_tier_1", period)
+        tier2 = tax_unit("taxable_social_security_tier_2", period)
+        total_taxable = tier1 + tier2
+
+        # Allocate total TOB based on tier 2 proportion
+        # Use where to handle division by zero
+        medicare_share = where(total_taxable > 0, tier2 / total_taxable, 0)
+
+        return total_tob * medicare_share

policyengine_us/variables/gov/ssa/revenue/tob_revenue_oasdi.py

@@ -0,0 +1,31 @@
+from policyengine_us.model_api import *
+
+
+class tob_revenue_oasdi(Variable):
+    value_type = float
+    entity = TaxUnit
+    definition_period = YEAR
+    label = "OASDI trust fund revenue from SS benefit taxation (tier 1)"
+    documentation = "Tax revenue from tier 1 (0-50%) Social Security benefit taxation credited to OASDI trust funds"
+    unit = USD
+
+    def formula(tax_unit, period, parameters):
+        """
+        Calculate OASDI trust fund revenue from tier 1 SS taxation.
+
+        Allocates total TOB revenue to OASDI based on tier 1's proportion
+        of total taxable SS.
+        """
+        # Get total TOB revenue
+        total_tob = tax_unit("tob_revenue_total", period)
+
+        # Get tier amounts
+        tier1 = tax_unit("taxable_social_security_tier_1", period)
+        tier2 = tax_unit("taxable_social_security_tier_2", period)
+        total_taxable = tier1 + tier2
+
+        # Allocate total TOB based on tier 1 proportion
+        # Use where to handle division by zero
+        oasdi_share = where(total_taxable > 0, tier1 / total_taxable, 0)
+
+        return total_tob * oasdi_share