Define workflow execution and checkpoint contracts

[shibayan]

Priority

P1

Goal

Introduce the minimum stable contracts required to execute and resume workflows without coupling the core runtime to a specific hosting or storage implementation.

Scope

• Define workflow input, output, event, state, checkpoint, and execution identifiers.
• Define cancellation and failure semantics.
• Define checkpoint storage abstraction and serialization requirements.
• Validate the design against official .NET and Python workflow behavior.

Acceptance criteria

• Contracts support multi-step execution and resumable checkpoints.
• Serialization is versionable and detects incompatible checkpoint data.
• Cancellation, retry, terminal failure, and completion states are explicit.
• Core contracts have unit tests and no Foundry-specific dependency.
• A short architecture document records compatibility decisions.