Use hourly wage in overtime premium for hourly workers

[MaxGhenis]

Summary

• use hourly_wage in fsla_overtime_premium for hourly workers when a positive hourly wage is available
• keep the existing earnings-based implied-rate fallback when hourly_wage is unavailable
• add policy regression cases and a changelog fragment

Why

hourly_wage is now available from the ORG imputation work, but the overtime premium formula was still reconstructing a base rate from annual earnings even for hourly workers. This wires the ORG-backed hourly wage into the formula in the guarded case where it represents the right straight-time rate.

Testing

• uv run python policyengine_us/tests/run_selective_tests.py --files policyengine_us/tests/policy/baseline/gov/irs/income/exemptions/fsla_overtime_premium.yaml
• uv run pytest policyengine_us/tests/core/test_org_input_variables.py -q
• uv run python -m py_compile policyengine_us/variables/household/income/person/fsla_overtime_premium.py
• direct engine-level checks via Simulation(..., situation=...) returned [5200.0] for the hourly-wage branch and [2727.272705078125] for the fallback branch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20b8599) to head (8eef86d).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7945   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           17        18    +1     
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+ Hits            17        18    +1     

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

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