[Py SDK] Draft transforms PoC: Python

[laurabelinfante]

Summary

Build a Python proof-of-concept for the plugin transformation interface. Validates the contract shape recommended in SPIKE #657 before locking the full implementation in both SDKs.

Issued separately from the TypeScript PoC to avoid SDK dependencies between them.

Acceptance criteria

• PoC implements the transformation contract shape recommended in SPIKE: Plugin transformation contract and SDK integration strategy #657
• PoC demonstrates bidirectional transformation between a sample source format and CommonGrants
• Findings and design questions captured to inform full SDK implementation