fix: correct 2020 surviving spouse standard deduction to $24,800

[MaxGhenis]

Summary

• Fixes the 2020 surviving spouse standard deduction value from $24,400 to $24,800
• Per 26 USC 63(c)(2)(A), surviving spouse filers receive the same standard deduction as joint filers
• Added tests verifying all 2020 standard deduction values by filing status

Fixes #7094

Test plan

• Added YAML test basic_standard_deduction.yaml covering all 5 filing statuses for 2020
• All tests pass: policyengine-core test policyengine_us/tests/policy/baseline/gov/irs/deductions/standard/basic_standard_deduction.yaml -c policyengine_us

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.07%. Comparing base (66ccff2) to head (8ce7eb1).
⚠️ Report is 430 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7279      +/-   ##
==========================================
+ Coverage   72.72%   75.07%   +2.35%     
==========================================
  Files        3757     3757              
  Lines       54115    54115              
  Branches      267      267              
==========================================
+ Hits        39353    40625    +1272     
+ Misses      14742    13467    -1275     
- Partials       20       23       +3     

Flag	Coverage Δ
unittests	75.07% <ø> (+2.35%)	⬆️

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