Model the 2025 Canada Workers Benefit secondary-earner exemption

[PavelMakarchuk]

Problem

PolicyEngine Canada does not model the secondary-earner exemption used in the
2025 Canada Workers Benefit calculation. A repository search for
secondary_earner and equivalent terms returns no implementation.

CRA Schedule 6 Step 2 allows an eligible-spouse family to deduct up to $16,386
of the lower earner's qualifying amount from adjusted family net income. Without
this step, the final CWB amount is not comparable to the 2025 CRA calculation.

The older general CWB implementation issue was #9.

Scope

• Add the 2025 secondary-earner exemption parameter.
• Identify the secondary earner from claimant and eligible-spouse working
  income.
• Cap the exemption by the secondary earner's working income, adjusted net
  income, and the published maximum.
• Subtract the exemption from adjusted family net income before applying the
  basic CWB and disability-supplement reductions.
• Preserve an explicit pre-exemption AFNI variable or otherwise expose both
  legal calculation boundaries.

Acceptance criteria

• No eligible spouse produces a zero exemption.
• The lower earner is selected correctly when claimant and spouse incomes
  differ.
• The exemption respects the working-income, adjusted-net-income, and $16,386
  caps.
• End-to-end tests cover the standard schedule and the Alberta, Quebec, and
  Nunavut variants.

Official source:
https://www.canada.ca/content/dam/cra-arc/formspubs/pbg/5000-s6/5000-s6-25e.pdf