Revert labor supply elasticity age heterogeneity

[PavelMakarchuk]

Revert age heterogeneity in labor supply response elasticities

Description:

Summary

This PR reverts the changes from version 1.426.0 (released 2025-10-28), which added age heterogeneity to labor supply response elasticities using a multiplier approach.

This is a selective revert - it only removes the labor supply elasticity changes while keeping all other recent updates from master (currently v1.432.7).

Changes

• Reverts commit 8e95b314b5 - "Add the 65+ age heterogeneity to US LSRs (Add the 65+ age heterogeneity to US LSRs #6681)"
• Restores simpler parameter structure: flat files instead of nested directories
• Parameters affected:
  • gov.simulation.labor_supply_responses.elasticities.income
  • gov.simulation.labor_supply_responses.elasticities.substitution

What's NOT Changed

This PR preserves all other recent work including:

• Bug fixes (Lifeline, CA CARE, Pell Grant, Head Start)
• State program updates (VA rebate, GA tax credit, etc.)
• Federal TANF infrastructure
• Rhode Island exemption fixes
• All other improvements from v1.426.1 through v1.432.7

Files Changed

16 files:

• Parameter structure simplified (removed age multiplier subdirectories)
• Variable formulas restored to previous implementation
• Tests updated to reflect simpler structure

Testing

✅ System loads successfully
✅ Basic simulations work correctly
✅ Parameter structure verified

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6c8cb6a) to head (66a73e5).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##           master     #6821       +/-   ##
============================================
+ Coverage   72.92%   100.00%   +27.07%     
============================================
  Files        3252         2     -3250     
  Lines       46840        38    -46802     
  Branches      243         3      -240     
============================================
- Hits        34159        38    -34121     
+ Misses      12667         0    -12667     
+ Partials       14         0       -14     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+27.07%)	⬆️

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