Harden stage contracts for initial harness engineering

[anth-volk]

This issue tracks the initial stage-contract hardening work for the US data harness engineering branch.

The implementation adds a stricter semantic contract surface for canonical data stages without wiring it into runtime orchestration yet.

Scope:

• Add shared canonical JSON helpers for deterministic metadata serialization.
• Make stage-contract constructors and JSON loading share the same validation expectations.
• Reject non-standard JSON constants such as NaN and infinity for contract metadata.
• Add canonical five-stage IDs, contract-type mappings, and substage membership checks.
• Preserve legacy CPS package-level lazy exports where practical to avoid import churn.
• Keep stage/substage examples in tests only, with validation that they match the canonical shape.

Out of scope:

• Runtime migration from step manifests to stage contracts.
• Moving stage folders or refactoring Modal orchestration.
• Conditional rerun/reuse implementation.
• Generated documentation publication.

Validation:

• Focused stage-contract, canonical JSON, run-context, TRO, step-manifest, and CPS export tests pass.
• Ruff passes on touched files.