Declare US spec variable operations

[MaxGhenis]

Summary

• pin microplex-us to merged Microplex spec-operation vocabulary commit 490c717
• add mp_spec.operation to all 278 variable manifest rows in specs/us-2024.yaml
• make Python provenance explicitly temporary by requiring every row to name a generic operation kind
• record current operation debt: 116 open_decision rows, plus impute/passthrough/takeup/structural/geoid/derive rows

Tests

• 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 tests/specs/test_us_2024_spec.py
• git diff --check