Add US L0 refit H5 reconstruction utility

[MaxGhenis]

Summary

• add a reusable US L0/refit H5 reconstruction utility that attaches saved post-L0 refit weights to an existing base support H5
• emit a content-addressed reconstruction manifest with base H5, weight NPZ, output H5, selected counts, weight sum, copied Populace attrs, and saved calibration metadata
• route the fiscal release builder's L0/refit export path through the same shared helper so fresh release builds and saved-weight reconstruction use one attachment implementation

Local reconstruction performed

uv run --package populace-build --extra us python tools/export_us_l0_refit_h5.py \
  --base-h5 /Users/maxghenis/.codex-worktrees/populace-historical-asec-pooling-20260629/out/pooled-asec-puf-support-three-year-20260629/base/base_populace_us_2024_puf_support.h5 \
  --weights-npz /Users/maxghenis/maria-l0-review/l0-paper-full-surface/runs/fixed-lambda-3asec-share0p8-e1500-matched-baselines-20260630/weights/informed_l0_refit_seed0_budget57240_l02p369e-06.npz \
  --output-h5 out/sparse-default-release-20260701/artifacts/populace_us_2024.h5 \
  --summary-json out/sparse-default-release-20260701/artifacts/populace_us_2024.l0_refit_export_summary.json

Final local artifact summary:

• output H5: out/sparse-default-release-20260701/artifacts/populace_us_2024.h5
• output SHA256: f028c2ee15b011a678586f5da024bd0dc2e10a5a8d6a9eef91235f34cc393c37
• base H5 SHA256: ec290055a1856e8528b13818e506501f160398b8660f67a3edc6bbea869fbe08
• weight NPZ SHA256: 3c2a872c7e624218f9fe2aa920210419c23c4b5b9c33cb38455f5f04fe3e3e16
• households: 57,240
• people: 166,302
• tax units: 79,736
• states: 51
• congressional districts: 436
• household weight sum: 134,690,323.3402448
• saved refit loss metadata: 0.04740850352691568 on 32,633 targets
• copied CD attrs: populace_congressional_district_vintage_target=119th_congress, populace_congressional_district_vintage_crosswalk_sha256=a61553933a585d23365bef0602328ff347a0ae00f35e8499516f0eef069ba17d

HDF5 bytes are not assumed deterministic across repeated writes; the manifest records the exact produced artifact hash.

Tests

• uv run ruff format packages/populace-build/src/populace/build/us_runtime/l0_refit_export.py packages/populace-build/tests/test_us_l0_refit_export.py tools/export_us_l0_refit_h5.py tools/build_us_fiscal_refresh_release.py
• uv run ruff check packages/populace-build/src/populace/build/us_runtime/l0_refit_export.py packages/populace-build/tests/test_us_l0_refit_export.py tools/export_us_l0_refit_h5.py tools/build_us_fiscal_refresh_release.py
• uv run pytest packages/populace-build/tests/test_us_l0_refit_export.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py::test_l0_refit_export_subsets_clean_base_frame
• local H5 load/provenance check against policyengine_us.data.USSingleYearDataset