Add 2025 values for Maine Sales Tax Fairness Credit

[PavelMakarchuk]

Summary

• Adds 2025-01-01 entries to amount/base.yaml ($155 single, $220 joint/HoH/surviving spouse) and reduction/start.yaml ($25,450 single, $38,200 HoH, $50,950 joint/surviving spouse), matching the 2025 Schedule PTFC/STFC line 17 table.
• Adds an integration test reproducing the MFJ-with-2-deps case from policyengine-taxsim Add NYS EITC and longer history for MA rental deduction #873 ($52,630 ME total income → $240 credit).
• PR Update Maine income tax parameters for 2025 #7384 updated other Maine 2025 parameters but skipped the STFC; this fills that gap.

Fixes #8262

Test plan

• policyengine-core test policyengine_us/tests/policy/baseline/gov/states/me/tax/income/credits/sales_tax_fairness_credit -c policyengine_us — 11 passed
• make format clean
• Manual formula trace: max_credit ($220 + $60) − ceil(($52,630 − $50,950) / $1,000) × $20 = $280 − $40 = $240
• Values cross-checked against page 4 of the 2025 Schedule PTFC/STFC PDF

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.45%. Comparing base (8cb60e7) to head (2e871ba).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #8266       +/-   ##
============================================
- Coverage   100.00%   70.45%   -29.55%     
============================================
  Files            3     4737     +4734     
  Lines           63    68915    +68852     
  Branches         0      348      +348     
============================================
+ Hits            63    48551    +48488     
- Misses           0    20364    +20364     

Flag	Coverage Δ
unittests	70.45% <ø> (-29.55%)	⬇️

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