Add South Carolina homestead property tax exemption

[daphnehanse11]

Summary

• Adds South Carolina's Homestead Exemption for senior, disabled, and blind homeowners.
• Models the exempt assessed value and property-tax reduction for the first ,000 of residence value.
• Adds focused eligibility, exemption, and reduction tests.

Closes #8201.

Tests

• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/sc/tax/property/homestead_exemption --batches 1
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/sc/tax/property/homestead_exemption
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (5174eca).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8311   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           63        39   -24     
=========================================
- Hits            63        39   -24     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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