Harden data pipeline against corrupted dataset uploads

[MaxGhenis]

Summary

Hardens the data pipeline with 5 layers of defense to prevent corrupted datasets from being uploaded to HuggingFace — the bug class from PR #569 where employment_income_before_lsr was dropped, zeroing all income and inflating poverty to 40%.

Defense layers

1. Pre-upload validation gate (upload_completed_datasets.py): File size, H5 structure, and aggregate stats checks run before ANY upload
2. Post-generation assertions (enhanced_cps.py, small_enhanced_cps.py): Weight validation, critical variable checks, output file size verification
3. CI workflow safety (reusable_test.yaml): Upload gated on success(), pre-upload H5 validation step
4. Makefile hardening: Sparse file swap validates existence/size before and after
5. Comprehensive tests: Sparse dataset sanity tests, direct employment income checks, file size regression test

How each layer would have caught PR #569's bug

Layer	Would catch?	How
Pre-upload validation	Yes	File was 14MB (threshold: 100MB), employment_income_before_lsr was empty
Post-generation assertions	Yes	create_sparse_ecps() output missing critical variables
CI workflow	Yes	H5 check catches missing employment_income_before_lsr
Makefile swap	Yes	Sparse file didn't exist (wrong filename), swap would fail
Tests	Yes	test_ecps_employment_income_positive catches $0 employment income

Test plan

• CI lint/format passes
• Smoke tests pass
• Full data build + sanity tests pass (will run on merge to main)
• Verify pre-upload validation blocks upload when given a bad file

🤖 Generated with Claude Code

[MaxGhenis]

CI status

5/6 checks pass (lint, smoke test, changelog, uv.lock, check-fork). The Test/test failure is expected — the full Modal data build produces datasets where employment_income = 0 (the known corruption issue), and our new sanity tests correctly catch this:

FAILED test_ecps_mean_employment_income_reasonable - Mean employment income = $0, expected $15k-$80k
FAILED test_ecps_poverty_rate_reasonable - Poverty rate = 42.3%, expected 5-25%
FAILED test_sparse_employment_income_positive - employment_income sum is 0.00e+00, expected > 5T

This is the hardening working as designed — these tests would have caught the original bug before it reached production. The tests will pass once the underlying data pipeline issue is resolved (separate from this PR).

This PR should be merged to get the safety nets in place. The test failures validate that the hardening is effective.