Drop temporary CPS take-up anchors from H5 outputs

[MaxGhenis]

Summary

• delete temporary SNAP/SSI take-up source anchors from persisted CPS H5 outputs after append-style saves
• reject temporary or retired reported/source variables during dataset upload validation
• add an eCPS-only data-build path that can skip source-imputed CPS and small enhanced CPS when only enhanced_cps_2024.h5 is needed
• checkpoint enhanced_cps clone diagnostics alongside the H5 so eCPS-only runs can validate and stage the sidecar
• record skipped Phase 5 in the Stage 1 contract and exclude stale small enhanced CPS artifacts from eCPS-only upload/promote

Where this broke

The prior production data run was Run Pipeline 25751771946, for commit d4871933 / version 1.112.3. That GitHub job completed because it only launched the Modal pipeline, but the Modal eCPS checkpoint from that run contained the temporary source anchors snap_reported and ssi_reported.

With this PR's validator, that checkpointed H5 fails before upload:

Validation failed for enhanced_cps_2024.h5:
  - Dataset contains temporary or retired variables: snap_reported, ssi_reported

The root cause is that Dataset.save_dataset appends/overwrites H5 keys but does not delete keys that were popped from the in-memory dict.

Tests

• uv run ruff format policyengine_us_data/datasets/cps/cps.py policyengine_us_data/storage/upload_completed_datasets.py policyengine_us_data/stage_contracts/dataset_build.py modal_app/data_build.py tests/integration/test_cps_generation.py tests/unit/test_upload_completed_datasets.py tests/unit/test_modal_data_build.py tests/unit/test_dataset_build_stage_contract.py
• uv run ruff check policyengine_us_data/datasets/cps/cps.py policyengine_us_data/storage/upload_completed_datasets.py policyengine_us_data/stage_contracts/dataset_build.py modal_app/data_build.py tests/integration/test_cps_generation.py tests/unit/test_upload_completed_datasets.py tests/unit/test_modal_data_build.py tests/unit/test_dataset_build_stage_contract.py
• uv run pytest tests/integration/test_cps_generation.py::test_add_takeup_removes_temporary_source_anchors_from_saved_h5 tests/integration/test_cps_generation.py::test_drop_persisted_dataset_variables_removes_stale_h5_keys tests/unit/test_upload_completed_datasets.py::test_validate_dataset_rejects_temporary_reported_source_variables -q
• uv run pytest tests/unit/test_dataset_build_stage_contract.py tests/unit/test_modal_data_build.py tests/unit/test_upload_completed_datasets.py tests/integration/test_cps_generation.py tests/unit/datasets/test_cps_takeup.py -q