Durable Execution SDK Specification

Provide a language SDK specification for developers to build their own SDKs and establish conformance testing.

The spec will live in this repo.

The specification defines the behavior that any conforming SDK implementation must exhibit. It covers the execution model, checkpointing semantics, callback protocol, parallel execution guarantees, and error propagation rules. SDK authors use this specification as the authoritative reference when implementing or verifying an SDK in any language.

This section will eventually also house a test specification or test suite that prospective Durable Execution SDKs must pass.

Prior art:
https://github.com/aws/aws-durable-execution-sdk-js/pull/369