Fix KY family size tax credit at 1.33 boundary

[DTrim99]

Summary

KY's Schedule ITC publishes Family Size Tax Credit thresholds in whole dollars and rounds 0.5-cent fractions up to the next whole dollar. PE's family_size/rate.yaml encoded the breakpoints as exactly 1.27 and 1.33, leaving a sub-dollar window where MGI lands between the share threshold and KY's published cutoff — resulting in PE returning a 0% rate instead of 10% (or 20% instead of 30%) at the boundary.

For 2025 family of 4 (FPG = $32,150):

• KY publishes "10% credit if MGI not over $42,760"
• $42,760 / $32,150 = 1.330016, not 1.33
• A household with true MGI = $42,759.88 (e.g. issue #852) gets share = 1.330012 in PE's float32, which exceeds the 1.33 bracket → 0% (TaxAct: 10%, $91 credit)

Fix bumps the two affected thresholds to 1.2701 and 1.3301. The other brackets (1.04, 1.08, 1.12, 1.16, 1.20, 1.24, 1.30) land on whole dollars for all KY family sizes and are unchanged.

Test plan

• Existing 4 KY FSTC rate tests still pass
• 3 new boundary tests (just below cutoff with cents, exactly at $42,760, clearly above) pass
• End-to-end check on issue 852 inputs: PE now returns FSTC = $91.04 and KY tax = $819.36, matching TaxAct ($91 / $819)
• CI green

Reported in PolicyEngine/policyengine-taxsim#852.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.47%. Comparing base (623a65c) to head (ca21cfa).
⚠️ Report is 152 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #8251       +/-   ##
============================================
- Coverage   100.00%   70.47%   -29.53%     
============================================
  Files           12     4726     +4714     
  Lines          173    68743    +68570     
  Branches         4      347      +343     
============================================
+ Hits           173    48450    +48277     
- Misses           0    20292    +20292     
- Partials         0        1        +1     

Flag	Coverage Δ
unittests	70.47% <ø> (-29.53%)	⬇️

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