Add labor supply elasticity age heterogeneity

[PavelMakarchuk]

Summary

This PR re-adds the age heterogeneity feature for labor supply response elasticities that was previously reverted. This implements the multiplier approach for age-based
differences in labor supply responses.

Context

• Original implementation: PR Add the 65+ age heterogeneity to US LSRs #6681 (v1.426.0, released 2025-10-28)
• Reverted in: PR Revert labor supply elasticity age heterogeneity #6821 (to address potential issues)
• This PR: Re-implements the same feature for further testing and evaluation

Changes

Parameter Structure:

• Restores nested directory structure for elasticity parameters
• Adds age threshold and multiplier parameters for income and substitution elasticities
• Age threshold: 65 years
• Multiplier approach allows different elasticities for people above/below threshold

Files Changed (16 total):

• parameters/gov/simulation/labor_supply_responses/elasticities/income/* - Age-adjusted income elasticity params
• parameters/gov/simulation/labor_supply_responses/elasticities/substitution/* - Age-adjusted substitution elasticity params
• variables/gov/simulation/labor_supply_response/*.py - Updated variable formulas
• Test files updated to reflect new structure

Technical Details

The age heterogeneity uses a multiplier approach where:

• Base elasticity values are defined
• Age threshold is set (default: 65)
• Multiplier is applied for individuals above the threshold
• This allows different labor supply responses for older vs. younger workers

Testing

✅ System loads successfully
✅ Parameter structure verified
✅ All files restored from original implementation

Purpose

This draft PR allows for:

1. Further testing of the age heterogeneity feature
2. Evaluation of its impact on policy simulations
3. Refinement before final merge decision

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.00%. Comparing base (66a73e5) to head (295d9ee).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...ulation/labor_supply_response/income_elasticity.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #6830      +/-   ##
===========================================
- Coverage   100.00%   92.00%   -8.00%     
===========================================
  Files            2        2              
  Lines           38       50      +12     
  Branches         3        5       +2     
===========================================
+ Hits            38       46       +8     
- Misses           0        2       +2     
- Partials         0        2       +2     

Flag	Coverage Δ
unittests	92.00% <90.90%> (-8.00%)	⬇️

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