Gate US spec variable provenance

[MaxGhenis]

Summary

• add temporary provenance entries in us-2024.yaml for every frozen eCPS export-contract variable plus every declared imputation variable
• explicitly flag net_worth as an open direct-SCF-vs-component decision and mark keep-CPS collision variables, SCF components, reported health leaves, takeup inputs, and geography exports
• wire spec.variables coverage into the fast export-column CLI and mp300k column_contract gate so required/exported or declared-imputation variables cannot silently fall out of the spec
• pin microplex to Add temporary variable provenance to spec DSL microplex#39 commit cad505289da23e1e7a5eded3c67a248cd8d1b8e4

Depends on PolicyEngine/microplex#39.

Validation

• env -u UV_FROZEN uv run --python 3.13 --extra dev pytest tests/specs/test_us_2024_spec.py tests/pipelines/test_check_export_columns.py tests/pipelines/test_mp300k_artifact_gates.py -q
• env -u UV_FROZEN uv run --python 3.13 --extra dev ruff check src/microplex_us/pipelines/check_export_columns.py src/microplex_us/pipelines/mp300k_artifact_gates.py tests/specs/test_us_2024_spec.py tests/pipelines/test_check_export_columns.py tests/pipelines/test_mp300k_artifact_gates.py
• git diff --check

Reviewer note: much of the per-variable provenance is intentionally marked temporary and broad. The new gate prevents omissions; later source-lane PRs should replace broad rows with exact executable source rules and narrower file:line evidence.