Add calibration warm-start and target-frame checkpoints

[MaxGhenis]

Summary

• let calibrate() start from validated prior household weights while preserving the original base weights for mass conservation, caps, diagnostics, and provenance
• add --warm-start-calibration-npz to the US fiscal refresh builder
• add a full target-frame checkpoint (target_frame_checkpoint.h5) so reruns can skip PE target/materialization work once the post-source target frame has been built

Notes

• The existing per-reform target-materialization cache remains content-addressed by build commit for low-level JCT vectors.
• The new target-frame checkpoint is keyed by the data/registry/runtime inputs, not the git commit, so warm-start-only code changes do not force rematerialization.
• Warm-start is rejected with L0 pruning / target_records until gate-state persistence exists.

Tests

• uv run --python 3.13 --package populace-build --extra us --group dev ruff check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-calibrate/src/populace/calibrate/solve.py packages/populace-calibrate/tests/test_solve.py
• uv run --python 3.13 --package populace-build --extra us --group dev ruff format --check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-calibrate/src/populace/calibrate/solve.py packages/populace-calibrate/tests/test_solve.py
• uv run --python 3.13 --package populace-build --extra us --group dev pytest packages/populace-calibrate/tests/test_solve.py -q packages/populace-build/tests/test_us_fiscal_refresh_builder.py -q