Workflow SDK now supports inflight cancellation

The Workflow SDK 5 beta now supports the standard AbortController and AbortSignal APIs across workflow and step boundaries. Create a controller inside a workflow, pass its signal into steps, and cancel in-flight operations using the same API fetch already uses. The signal stays durable across suspensions and deterministic replay. Cancellation is cooperative; steps have to inspect the signal or pass it to an API that supports AbortSignal. Use cases include stopping slow steps on timeout, cancelling remaining requests after first success, threading signals through pipelines, or cancelling parallel work on external condition changes.