Reintroduce age-specific labor supply response multipliers

[MaxGhenis]

Summary

• reintroduce age heterogeneity for labor supply responses without changing the legacy scalar income elasticity parameter path
• add age threshold and multiplier parameters for substitution responses and parallel sibling parameters for income responses
• harden the labor-supply zero-response guard so nonzero primary or secondary substitution elasticities are not skipped

Why this shape

The reverted attempt changed the elasticities.income parameter from a scalar into a nested node and also regressed the secondary-earner substitution path. This version avoids both rollout hazards:

• gov.simulation.labor_supply_responses.elasticities.income stays scalar and backward-compatible
• secondary substitution elasticities still resolve and now have direct coverage

Testing

• pytest -q policyengine_us/tests/core/test_labor_supply_behavioral_response_guard.py policyengine_us/tests/core/test_behavioral_response_measurements.py
• python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/simulation/labor_supply_response/income_elasticity.yaml policyengine_us/tests/policy/baseline/gov/simulation/labor_supply_response/substitution_elasticity.yaml policyengine_us/tests/policy/baseline/gov/simulation/labor_supply_response/integration.yaml -c policyengine_us
• pytest -q policyengine_us/tests/microsimulation/test_behavioral_structural.py policyengine_us/tests/microsimulation/test_lsr_cg_interaction.py (skipped without RUN_HEAVY_TESTS=1)

Closes #6265.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (b70c547) to head (b875aed).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7979      +/-   ##
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- Coverage   100.00%   97.22%   -2.78%     
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  Files            1        3       +2     
  Lines           13       72      +59     
  Branches         0        5       +5     
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+ Hits            13       70      +57     
- Misses           0        1       +1     
- Partials         0        1       +1     

Flag	Coverage Δ
unittests	97.22% <100.00%> (-2.78%)	⬇️

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