Remove national L0 penalty

[MaxGhenis]

Summary

• Set national fit defaults to lambda_l0=0 and route that path through a dense positive log-weight optimizer, so national calibration does not import or construct L0/HardConcrete gates.
• Keep local/regional L0 behavior intact for local-area calibration.
• Remove the national Modal Makefile override that forced positive --lambda-l0.
• Update calibration docs, generated pipeline docs, and notebook language; add regression coverage for the dense national path and Makefile contract.
• Add the required changelog fragment and refresh the policyengine-us pin to the PyPI-current 1.722.4 so PR freshness checks pass.

Tests

• uv run --python 3.13 pytest tests/unit/calibration/test_unified_calibration.py tests/unit/test_makefile_contracts.py tests/unit/fit_weights/test_specs.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_remote_calibration_runner.py
• uv run --python 3.13 --no-sync --with modal --with pytest pytest tests/unit/test_pipeline.py
• uv run --python 3.13 ruff format --check .
• env -u UV_FROZEN uv lock --locked
• uv run --python 3.13 python .github/scripts/check_policyengine_us_dependency.py --mode fail
• env -u UV_FROZEN uv run --python 3.13 towncrier check --compare-with origin/main
• git diff --check
• notebook/generated JSON parse check

Review

• /cycle read-only review found and fixed stale Makefile/docs issues.
• Follow-up review reported no actionable findings.

[MaxGhenis]

Closing as part of archiving this repository — Populace (PolicyEngine/populace) is its replacement and the certified US dataset since June 2026; see PolicyEngine/populace#204. This PR predates the migration and will not be merged. This topic continues in Populace's calibration gate suite and PolicyEngine/populace#212's closing analysis. Published artifacts remain available but frozen.