Summary

• Adds Texas school district residence homestead exemption parameters for the current $140,000 general school-district exemption and additional $60,000 age-65-or-disabled exemption.
• Adds eligibility and exemption-value variables, including disabled and surviving-spouse eligibility, with combined exempt value capped by assessed property value.
• Adds focused tests for general, additional, and total school district homestead exemption values.

Closes #8204.

Value Review

• Texas Comptroller lists a $140,000 residence homestead exemption for school taxes for all residence homestead owners.
• Texas Comptroller lists an additional $60,000 residence homestead exemption for school taxes for age-65-or-disabled residence homestead owners, making the modeled combined maximum $200,000.
• TEA's Tax Year 2025 guidance says SB 4 and SB 23 raise the general exemption from $100,000 to $140,000 and the age-65-or-disabled exemption from $10,000 to $60,000 for TY 2025.
• The surviving-spouse age threshold is 55 per the Comptroller's summary of Tax Code Section 11.13(q). This model approximates continuation eligibility with available filing-status inputs.

Note: this PR models exempt value only, not a property-tax reduction amount, because the statewide exemption applies to school district taxes and PolicyEngine currently has a single real-estate-tax input rather than a school-tax component.

Sources:

• Texas Comptroller, Property Tax Exemptions: https://comptroller.texas.gov/taxes/property-tax/exemptions/
• Texas Education Agency, Tax Year 2025 Maximum Compressed Tax Rates: https://tea.texas.gov/sites/default/files/taa-2025-06-12-tax-year-2025-maximum-compressed-tax-rates.pdf#page=2

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/states/tx/tax/property/school_district_homestead_exemption -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py -q
• uv run --extra dev ruff check policyengine_us/variables/gov/states/tx/tax/property/school_district_homestead_exemption