Add 2026 tax year and automation tool for future updates

[rahulaggarwal965]

Summary

• Add 2026 federal payroll withholding tables from IRS Publication 15-T for all 6 table combinations (single, married, HoH × standard, multiple jobs)
• Add 2026 Social Security wage base ($184,500)
• Fix 2025 married standard_schedule — 5 bracket max values had a missing trailing digit (e.g. 40949.9 → 40949.99)
• Add tools/update_tax_year.py — automates adding future tax years by:
  • Downloading and parsing IRS Pub 15-T PDFs with pdfplumber
  • Fetching the SS wage base from the Federal Register API
  • Validating extracted data against structural invariants
  • Updating all source files programmatically
• Add 50 tests covering parsing, validation, code generation, SS wage base fetching, and integration verification against 2024/2025/2026 PDFs

Usage (future tax years)

uv run --group tools python tools/update_tax_year.py

Test plan

- uv run --group test python -m pytest tests/ — all existing tests pass
- uv run --group tools --group test python -m pytest tests/tools/ -v — all 50 tool tests pass (downloads PDFs from IRS, fetches SS wage base from Federal Register)

Might be worth putting this in some sort of automation?