CI workflow takes 2.5+ hours due to sequential data builds

[baogorek]

Problem

The PR code changes workflow is taking 2.5+ hours to complete, far longer than the expected ~45 minutes. The bottleneck is in the Modal data build step (modal_app/data_build.py).

Root Cause

The Modal data build runs everything sequentially with no parallelization:

First pass (with TEST_LITE):

1. Download prerequisites
2. uprating.py
3. acs.py
4. cps.py
5. irs_puf.py
6. puf.py
7. extended_cps.py
8. enhanced_cps.py ← Likely the slowest (calibration/reweighting)
9. small_enhanced_cps.py

Second pass (LOCAL_AREA_CALIBRATION - runs full, not lite):

10. cps.py again
11. puf.py again
12. extended_cps.py again
13. create_stratified_cps.py

Then tests:

14. Local area calibration tests
15. Main test suite

Issues

1. No parallelization - All scripts run sequentially despite having 8 CPUs available
2. Duplicate builds - Several datasets are built twice (once for main, once for local area calibration)
3. enhanced_cps.py involves expensive optimization/calibration operations

Observed

• Run #21254423149 took 2+ hours
• The job appeared to hang/stall at times during the Modal step

Potential Solutions

• Parallelize independent data build steps
• Cache intermediate results between the two build passes
• Profile individual scripts to identify specific bottlenecks
• Consider whether both build passes are necessary for every PR