Use PA retirement-age threshold for employer pension exclusion

[DTrim99]

Fixes #9386.

Problem

pa_nontaxable_pension_income gated the Pennsylvania employer-pension exclusion on is_retired, which is hard-coded to age ≥ 65:

retired = person("is_retired", period)   # is_retired = age >= 65

But the parallel pa_nontaxable_retirement_distributions (IRA/401(k)/403(b)/SEP/Keogh) already uses PA's retirement_age_threshold = 59.5. 61 Pa. Code §101.6 treats federally qualified employee pension plans the same as IRAs, SEPs, and Keogh plans — all excluded once the recipient reaches the plan's retirement age — so the two should not use different ages. The PA-40 IN (2023, p.8) lists as not taxable: "Commonly recognized pension, old age, or retirement benefits paid after becoming eligible to retire, and retiring."

The mismatch over-taxed employer pensions for retirees aged 59.5–64.

Fix

Gate pa_nontaxable_pension_income on PA's retirement_age_threshold (59.5), matching pa_nontaxable_retirement_distributions.

Impact (TAXSIM #1165)

PA 2023 joint, primary 72 / spouse 64, no wages; $37,636 interest, $201,382 pension, $52,916 Social Security. The pension is split 50/50, and the 64-year-old's half was wrongly taxed:

• Before: PA AGI $138,327, tax $4,246.64
• After: PA AGI $37,636 (interest only), tax $1,155.42 — matches TaxAct (100% of pension + SS excluded)

Tests

• Updated the unit test to probe the 59.5 boundary: age 59 → taxable, age 60 → excluded, age 67 → excluded.
• PA income-tax suite passes (51 tests).

🤖 Generated with Claude Code

[DTrim99]

@PavelMakarchuk flagging a small convention point: this changes the PA employer-pension exclusion age from 65 (the generic is_retired default) to PA's own retirement_age_threshold of 59.5. The justification is that 61 Pa. Code §101.6 treats federally qualified employee pension plans the same as IRAs/SEP/Keogh (which we already exclude at 59.5 via pa_nontaxable_retirement_distributions), so keeping employer pensions at 65 was an internal inconsistency. It reverses the old age-60→taxable unit-test case, which encoded the age-65 assumption rather than an external PA source. Happy to adjust if you'd prefer a different threshold.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (837a1ad) to head (1b48dc5).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9387   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            3         1    -2     
  Lines           46        15   -31     
=========================================
- Hits            46        15   -31     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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