Add Louisiana Special Assessment Level freeze for seniors

## Summary

Add Louisiana's Special Assessment Level freeze for older homeowners.

## Why add this

- Louisiana provides statewide senior property tax protection through the Special Assessment Level.
- It appears missing from `policyengine-us`.

## Modeling notes

- This is an assessment freeze, not a direct rebate.
- It belongs in the long-run comparison inventory, but it is not a strong first-pass modeling target.
- Full implementation would likely need base-year assessed value mechanics.

## Likely inputs

- `age`
- homeownership / homestead status
- household income
- `assessed_property_value`
- base-year assessed value or prior assessed value

## Sources

- https://house.louisiana.gov/housefiscal/DOCS_TAXREV/2104_Louisiana%20Property%20Tax%20Basics.pdf
- https://my.aarpfoundation.org/property-taxes/taxpayer-states/louisiana/