Implement tier-separated trust fund revenue (OASDI vs Medicare HI)

[MaxGhenis]

Background

Trust fund revenue from SS benefit taxation needs to be properly separated by tier:

• Tier 1 (0-50% taxation) → OASDI trust funds
• Tier 2 (50-85% taxation) → Medicare HI trust fund

Currently we have tob_revenue_total which calculates total revenue correctly using branching + neutralization.

Problem

Proportional allocation approach (allocating total based on tier amounts) is not correct because:

• Different tiers may be taxed at different marginal rates
• Need to directly measure marginal tax impact of each tier separately

Solution

Implement separate branching for each tier:

class tob_revenue_oasdi(Variable):
    def formula(tax_unit, period, parameters):
        sim = tax_unit.simulation
        income_tax_with = tax_unit('income_tax', period)
        
        # Branch and neutralize ONLY tier 1
        branch = sim.get_branch('tob_tier1', clone_system=True)
        branch.tax_benefit_system.neutralize_variable('taxable_social_security_tier_1')
        
        # Delete all calculated variables
        for var in branch.tax_benefit_system.variables:
            if var not in branch.input_variables:
                branch.delete_arrays(var)
        
        income_tax_without_tier1 = branch.tax_unit('income_tax', period)
        del sim.branches['tob_tier1']
        
        return income_tax_with - income_tax_without_tier1

Similar for tob_revenue_medicare_hi with tier 2.

Validation Needed

Test that tier1 + tier2 = total (within rounding).

References

• PR Add trust fund revenue variables with LSR recursion fix and tier separation #6750 has tob_revenue_total working
• Tier variables: taxable_social_security_tier_1, taxable_social_security_tier_2