Core plumbing for YAML workflow support:
- Add output_data field to ExecutionResult for step output capture
- Executor merges output_data into workflow.data after step completion
- Add register_step_factory(key, factory) to WorkflowHost for
dynamic step registration by external crates like wfe-yaml
- Add info!-level tracing to workflow executor: logs each execution
round, each step run (with type and name), step completion, and
workflow completion
- WorkflowHost.start() now auto-registers all built-in primitive step
types so users don't need to register them manually
- Add #[derive(Default)] to all primitive steps and PollEndpointConfig
- Add tracing-subscriber to wfe crate for the pizza example
- Pizza example now shows full step-by-step execution logs
