Add South Carolina homestead exemption for seniors

[daphnehanse11]

Summary

Add South Carolina's homestead exemption for seniors.

Why add this

• South Carolina has statewide senior homeowner property tax relief.
• It appears missing from policyengine-us.

Modeling notes

• This is a fair-market-value exemption rather than a direct refund.
• Useful for the comparison inventory, but a weaker first coding target than direct credits.

Likely inputs

• age
• homeownership / legal residence status
• assessed_property_value
• real_estate_taxes

Sources

• https://dor.sc.gov/exempt-property
• https://my.aarpfoundation.org/property-taxes/taxpayer-states/south-carolina/