Publish stable worker protocol contracts for embedded deployments

[ethan-tyler]

Summary

Embedded adopters need a versioned worker protocol surface from Arco instead of reimplementing dispatch and callback contracts downstream.

Severity: high

Downstream evidence

We had to locally recreate:

• task envelope types
• started, heartbeat, and completed payloads
• heartbeat control responses
• task outcome and run status enums
• stable attempt ID helpers

Requested outcome

• Publish canonical serializable worker protocol types in an upstream crate or module
• Document header and auth expectations plus compatibility guarantees
• Add golden JSON fixtures or compatibility tests for the wire contract
• Provide minimal client and server examples for embedded orchestrator and worker setups

Why upstream

This is core Arco orchestration behavior. Leaving it to each integrator creates drift and makes upgrades risky.

[ethan-tyler]

More concrete downstream evidence from Daxis:

• We currently preserve compatibility with older completed callback payloads that omit result; see crates/daxis-common/src/workflow.rs.
• Our worker client also treats an empty heartbeat response body as implicit continue; see crates/daxis-workflow-worker/src/callbacks.rs.

That is wire-contract behavior, not product logic. A versioned upstream worker protocol would remove this class of downstream compatibility shims.