Add provincial tax credit programs

[vahid-ahmadi]

Fixes #528

Overview

This PR implements missing provincial tax credit and benefit programs identified in issue #528 as part of
the SPSD/M parity effort. The implementation adds 3 new provincial programs with comprehensive test
coverage.

Summary:

• Total programs in issue Implement provincial tax credit programs #528: 16
• Already implemented: 11 (69%)
• Newly implemented in this PR: 3 (19%)
• Not implemented: 2 (12%)
• Overall coverage: 14/16 (88%) ✅

Detailed Implementation

1. BC Sales Tax Credit

Purpose: Offsets sales tax impact for low-income BC residents

Implementation:

• Location: policyengine_canada/variables/gov/provinces/bc/tax/income/credits/bc_sales_tax_credit/
• Variables:
  • bc_sales_tax_credit.py - Main credit calculation
  • bc_sales_tax_credit_base.py - $75 per eligible adult
  • bc_sales_tax_credit_reduction.py - 2% reduction on income over threshold
• Parameters:
  • Base amount: $75 per person
  • Thresholds: $15,000 (single), $18,000 (couple)
  • Reduction rate: 2%
• Tests: 8 test cases covering eligibility, income thresholds, and reductions

2. Alberta Seniors Benefit

Purpose: Provides financial assistance to low-income Alberta seniors

Implementation:

• Location: policyengine_canada/variables/gov/provinces/ab/benefits/ab_seniors_benefit/
• Variables:
  • ab_seniors_benefit.py - Main benefit
  • ab_seniors_benefit_eligible.py - Age 65+ and receiving OAS
  • ab_seniors_benefit_base.py - Maximum amounts by marital status
  • ab_seniors_benefit_reduction.py - 15.61% income reduction
• Parameters:
  • Max benefit: $5,756 (2025), $3,792 (2024)
  • Thresholds: $33,410 (single), $54,640 (couple) for 2025
  • Reduction rate: 15.61%
• Tests: 10 test cases covering eligibility, income levels, and year-specific amounts

3. Quebec Solidarity Tax Credit

Purpose: Helps offset housing costs and QST for low/middle-income Quebec households

Implementation:

• Location: policyengine_canada/variables/gov/provinces/qc/tax/income/credits/solidarity/
• Variables:
  • qc_solidarity_tax_credit.py - Main credit aggregator
  • qc_solidarity_housing_component.py - Housing cost offset
  • qc_solidarity_qst_component.py - QST (sales tax) offset
• Parameters:
  • Housing: $814 (single), $1,222 (couple/single parent)
  • QST: $306 per person
  • Separate thresholds and reduction rates for each component
• Tests: 8 test cases covering family compositions and income levels

Files Changed

Variables: 10 new files
Parameters: 19 new files
Tests: 3 new files
Total: 32 new files

Programs Not Implemented

Manitoba Education Property Tax Credit

Reason: Requires is_homeowner variable that doesn't exist in the current system. Implementation would
require:

1. Adding is_homeowner variable to household/person entity
2. Defining how homeownership status is determined
3. Integration with existing tax/benefit calculations

This should be addressed in a separate PR focused on property-related variables.

Nova Scotia Affordable Living Tax Credit

Reason: Already implemented in the codebase at:
policyengine_canada/variables/gov/provinces/ns/tax/income/credits/nsaltc/ns_affordable_living_tax_credit.py

References

• Issue: Implement provincial tax credit programs #528
• CRA Official Benefits List: https://www.canada.ca/en/services/taxes/child-and-family-benefits.html
• BC Sales Tax Credit: https://www2.gov.bc.ca/gov/content/taxes/income-taxes/personal/credits/sales-tax
• Alberta Seniors Benefit: https://www.alberta.ca/alberta-seniors-benefit
• Quebec Solidarity Tax Credit: https://www.revenuquebec.ca/en/citizens/tax-credits/solidarity-tax-credit/