Fix fiitax to include Additional Medicare Tax

[PavelMakarchuk]

Summary

• PE's income_tax variable does not include Additional Medicare Tax (0.9% on wages above $200K single / $250K MFJ), but TAXSIM includes it in fiitax (per IRS Form 1040 Schedule 2)
• The existing code at lines 1099-1108 tried to add additional_medicare_tax to fiitax, but the else branch was dead code — fiitax was always already computed from the income_tax mapping, so the pass branch ran instead
• Fix: always compute additional_medicare_tax and add it to fiitax

Impact on CPS comparison (1000 records, with --disable-salt --assume-w2-wages):

• Total absolute error reduced by 44% ($12.1M → $6.7M)
• 115 records improved, 30 worsened (mostly from TAXSIM's own Additional Medicare Tax calculation differences)
• At $15 tolerance: 4 borderline records near the threshold break (PE correctly computes $85-$179 of Additional Medicare Tax that TAXSIM doesn't)
• At $500+ tolerance: +12 to +43 more matches

Test plan

• All 38 existing tests pass
• Verified against TAXSIM output for 1000 CPS records
• Confirmed PE's Additional Medicare Tax calculation matches IRS Schedule SE / Form 8959

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