Add South Dakota senior property tax relief programs

## Summary

Add South Dakota's statewide senior property tax relief programs, especially the Tax Refund for Senior Citizens and Citizens with Disabilities.

## Why add this

- South Dakota has multiple statewide senior-focused property tax relief programs.
- They appear missing from `policyengine-us`.

## Modeling notes

- First-pass focus, if any: the statewide tax refund.
- Lower-priority items: the Assessment Freeze for the Elderly & Disabled and the Property Tax Homestead Exemption / deferral.
- The tax refund is trickier than a pure property tax rebate because the benefit may be based on either sales tax or property tax.

## Likely inputs

- `age`
- disability status if disability path included
- household income
- homeownership / occupancy status
- `real_estate_taxes`
- consumption / sales tax information if full refund mechanics are modeled

## Sources

- https://dor.sd.gov/individuals/taxes/property-tax/relief-programs/
- https://my.aarpfoundation.org/property-taxes/taxpayer-states/south-dakota/