feat(wfe-core): add output_data to ExecutionResult and register_step_factory to host

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

wfe-core/src/models/execution_result.rs

@@ -26,6 +26,9 @@ pub struct ExecutionResult {
     pub branch_values: Option<Vec<serde_json::Value>>,
     /// Poll endpoint configuration for external service polling.
     pub poll_endpoint: Option<PollEndpointConfig>,
+    /// Output data to merge into workflow.data after step completion.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub output_data: Option<serde_json::Value>,
 }
 
 impl ExecutionResult {