Add South Carolina homestead property tax exemption

[daphnehanse11]

Summary

• Adds South Carolina's Homestead Exemption for senior, disabled, and blind homeowners.
• Models the exemption for the first $50,000 of fair market residence value, converted to a $2,000 assessed-value exemption using South Carolina's 4% legal-residence assessment rate.
• Adds focused eligibility, exemption, and reduction tests.
• Keeps the rule household-calculator-only for now; this PR does not wire the reduction into baseline property-tax-credit aggregates while assessed-property-value data readiness is unresolved.

Closes #8201.

Value Review

• South Carolina Department of Revenue says the Homestead Exemption exempts the first $50,000 of the fair market value of the dwelling place for qualifying homeowners.
• The same source states legal residences are assessed at 4%, so this PR applies the $50,000 exemption as $2,000 of assessed value before estimating the property-tax reduction.

Source:

• South Carolina Department of Revenue, Exempt Property: https://dor.sc.gov/property/exempt-property

Tests

• PYTHONPATH=. /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/sc/tax/property/homestead_exemption -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/sc/tax/property/homestead_exemption

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee0f91) to head (0cd4c7c).
⚠️ Report is 128 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8311       +/-   ##
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+ Coverage   78.72%   100.00%   +21.27%     
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  Files        4723         3     -4720     
  Lines       68774        41    -68733     
  Branches      340         0      -340     
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- Hits        54142        41    -54101     
+ Misses      14554         0    -14554     
+ Partials       78         0       -78     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+21.27%)	⬆️

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