Add eCPS export support parity gate

[MaxGhenis]

Summary

• extend the fast eCPS export checker with optional H5 support parity against a pinned eCPS baseline
• add a required mp300k artifact gate named export_support so presence-only export parity no longer passes release artifacts
• report unsupported eCPS-populated columns and write optional support diagnostics JSON

Why

Column parity alone let the #190 candidate pass with 246/246 required exports even though many eCPS-populated columns were all-zero or single-valued in MP. This gate requires MP to populate any required exported column that eCPS populates, while treating eCPS all-zero/single-valued columns as fillers unless explicitly exempted.

Validation

• 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/pipelines/test_check_export_columns.py tests/pipelines/test_mp300k_artifact_gates.py
• uv run --python 3.13 --extra dev ruff format --check src/microplex_us/pipelines/check_export_columns.py src/microplex_us/pipelines/mp300k_artifact_gates.py tests/pipelines/test_check_export_columns.py tests/pipelines/test_mp300k_artifact_gates.py
• uv run --python 3.13 --extra dev python -m pytest tests/pipelines/test_check_export_columns.py tests/pipelines/test_mp300k_artifact_gates.py
• ran the new support gate against the Fix rent and childcare export inputs #190 release-candidate H5 and latest local eCPS; it still passes 246/246 column presence but fails support parity with 75 unsupported eCPS-populated exports, writing diagnostics to the Fix rent and childcare export inputs #190 post_h5_checks directory