Add initial stage contract harness foundation

[anth-volk]

Fixes #913

Summary

• Add the initial stage-contract schema surface for the US data harness engineering work.
• Add canonical stage and substage identifiers with stage-to-contract-type validation.
• Add shared canonical JSON helpers and use them for stage contracts, fingerprints, TROs, step manifests, release manifests, and run contexts.
• Harden direct contract constructors so they match JSON-loading validation and normalize nested sequence fields.
• Preserve legacy CPS lazy export names and remove duplicate root exports.

Validation

• uv run --no-sync pytest tests/unit/test_stage_contracts.py tests/unit/test_trace_tro.py::test_canonical_json_bytes_match_serialize tests/unit/test_step_manifest.py::test_step_manifest_serialization_is_deterministic tests/unit/test_run_context.py tests/unit/datasets/test_cps_lazy_exports.py
• uv run --no-sync ruff check policyengine_us_data/stage_contracts policyengine_us_data/utils/canonical_json.py policyengine_us_data/utils/trace_tro.py policyengine_us_data/utils/step_manifest.py policyengine_us_data/utils/release_manifest.py policyengine_us_data/utils/run_context.py policyengine_us_data/datasets/cps/init.py policyengine_us_data/init.py tests/unit/test_stage_contracts.py tests/unit/datasets/test_cps_lazy_exports.py