feat(wfe-core): add step primitives for workflow control flow

12 step primitives implementing StepBody: DecideStep, IfStep, WhileStep,
ForEachStep, SequenceStep, DelayStep, WaitForStep, ScheduleStep,
RecurStep, PollEndpointStep, SagaContainerStep, EndStep.

Each primitive handles its state machine via persistence_data and
branch creation for container steps.

wfe-core/src/primitives/decide.rs

@@ -0,0 +1,54 @@
+use async_trait::async_trait;
+
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+/// A decision step that returns an outcome value for routing.
+pub struct DecideStep {
+    pub expression_value: serde_json::Value,
+}
+
+#[async_trait]
+impl StepBody for DecideStep {
+    async fn run(&mut self, _context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        Ok(ExecutionResult::outcome(self.expression_value.clone()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::ExecutionPointer;
+    use crate::primitives::test_helpers::*;
+    use serde_json::json;
+
+    #[tokio::test]
+    async fn returns_correct_outcome_value() {
+        let mut step = DecideStep {
+            expression_value: json!("route_a"),
+        };
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+        let workflow = default_workflow();
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+
+        let result = step.run(&ctx).await.unwrap();
+        assert!(result.proceed);
+        assert_eq!(result.outcome_value, Some(json!("route_a")));
+    }
+
+    #[tokio::test]
+    async fn returns_numeric_outcome() {
+        let mut step = DecideStep {
+            expression_value: json!(42),
+        };
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+        let workflow = default_workflow();
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+
+        let result = step.run(&ctx).await.unwrap();
+        assert!(result.proceed);
+        assert_eq!(result.outcome_value, Some(json!(42)));
+    }
+}