Backdate New Hampshire interest and dividends tax to 2020

[MaxGhenis]

Summary

• Extends NH interest and dividends tax parameters back to 2020
• The 5% rate and exemption amounts ($2,400 single/$4,800 joint, $1,200 additional exemptions) were unchanged from 2020-2022, so this is a simple backdate
• Adds 2020 reference to the 2020 DP-10 form

Test plan

• Added 2020 test case verifying 5% rate (2000 taxable income → $100 tax)
• All existing NH exemption tests pass (28 tests)

Closes #6301

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.16%. Comparing base (e9bb8c8) to head (8b1dc02).
⚠️ Report is 175 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #7273       +/-   ##
============================================
- Coverage   100.00%   71.16%   -28.84%     
============================================
  Files           14     3738     +3724     
  Lines          212    53839    +53627     
  Branches         0      266      +266     
============================================
+ Hits           212    38314    +38102     
- Misses           0    15523    +15523     
- Partials         0        2        +2     

Flag	Coverage Δ
unittests	71.16% <ø> (-28.84%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[DTrim99]

PR Review

🟢 Overall: Approve

This is a clean, straightforward backdate PR that extends NH interest and dividends tax parameters from 2021 to 2020.

✅ Validation Summary

Check	Result
Regulatory Accuracy	✅ Verified - 5% rate and exemption amounts consistent with phaseout schedule
Reference Quality	⚠️ 1 issue (see below)
Code Patterns	✅ No issues introduced
Test Coverage	✅ Adequate for backdate
CI Status	✅ Passing (codecov/project failure is acceptable)

🟡 Should Address

1. Missing 2020 references in exemption files: The PR adds 2020 DP-10 references to rate.yaml and in_effect.yaml, but 6 exemption parameter files are being backdated without adding the 2020 reference:

   • exemptions/amount/base.yaml
   • exemptions/amount/blind_addition.yaml
   • exemptions/amount/disabled_addition.yaml
   • exemptions/amount/old_age_addition.yaml
   • exemptions/disability_age_threshold.yaml
   • exemptions/old_age_eligibility.yaml

   Consider adding:

   - title: 2020 New Hampshire DP-10 Form
     href: https://www.revenue.nh.gov/forms/2021/documents/dp-10-2020-print.pdf#page=3

🟢 Verified

• ✅ Test calculation correct: $2,000 × 5% = $100
• ✅ Changelog entry present
• ✅ Parameter date changes are consistent across all files (2021-01-01 → 2020-01-01)
• ✅ Values unchanged for the backdate period (same as 2021)

---

🤖 Automated review by Claude Code

[DTrim99]

I would add the 2020 form to all parameters adjusted, but other than that I approve.