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/executor/result_processor.rs

@@ -4,10 +4,12 @@ use crate::models::{
     EventSubscription, ExecutionPointer, ExecutionResult, PointerStatus, WorkflowDefinition,
 };
 
-/// Outcome of processing an ExecutionResult: new pointers and optional subscriptions.
+/// Outcome of processing an ExecutionResult: new pointers, subscriptions, and output data.
 pub struct ProcessResult {
     pub new_pointers: Vec<ExecutionPointer>,
     pub subscriptions: Vec<EventSubscription>,
+    /// Output data to merge into workflow.data (from step's output_data field).
+    pub output_data: Option<serde_json::Value>,
 }
 
 /// Process an ExecutionResult and update the pointer accordingly.
@@ -113,6 +115,7 @@ pub fn process_result(
     ProcessResult {
         new_pointers,
         subscriptions,
+        output_data: result.output_data.clone(),
     }
 }
 

wfe-core/src/executor/workflow_executor.rs

@@ -214,6 +214,14 @@ impl WorkflowExecutor {
 
                     all_subscriptions.extend(process_result.subscriptions);
 
+                    // Merge output data into workflow.data.
+                    #[allow(clippy::collapsible_if)]
+                    if let Some(serde_json::Value::Object(out_map)) = process_result.output_data {
+                        if let serde_json::Value::Object(wf_map) = &mut workflow.data {
+                            wf_map.extend(out_map);
+                        }
+                    }
+
                     // Add new pointers.
                     for new_pointer in process_result.new_pointers {
                         workflow.execution_pointers.push(new_pointer);

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 {

wfe/src/host.rs

@@ -201,6 +201,17 @@ impl WorkflowHost {
         sr.register::<S>();
     }
 
+    /// Register a step factory with an explicit key and factory function.
+    /// Used by wfe-yaml and other dynamic step sources.
+    pub async fn register_step_factory(
+        &self,
+        key: &str,
+        factory: impl Fn() -> Box<dyn StepBody> + Send + Sync + 'static,
+    ) {
+        let mut sr = self.step_registry.write().await;
+        sr.register_factory(key, factory);
+    }
+
     /// Start a new workflow instance.
     #[tracing::instrument(
         name = "workflow.start",