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/sequence.rs

@@ -0,0 +1,78 @@
+use async_trait::async_trait;
+use serde_json::json;
+
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+/// A container step that executes its children sequentially.
+/// Completes when all children have finished.
+pub struct SequenceStep;
+
+#[async_trait]
+impl StepBody for SequenceStep {
+    async fn run(&mut self, context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        let mut scope = context.execution_pointer.scope.clone();
+        scope.push(context.execution_pointer.id.clone());
+
+        if context.workflow.is_branch_complete(&scope) {
+            Ok(ExecutionResult::next())
+        } else {
+            Ok(ExecutionResult::persist(json!({"children_active": true})))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::{ExecutionPointer, PointerStatus};
+    use crate::primitives::test_helpers::*;
+
+    #[tokio::test]
+    async fn children_complete_proceeds() {
+        let mut step = SequenceStep;
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+
+        let mut workflow = default_workflow();
+        let mut child = ExecutionPointer::new(1);
+        child.scope = vec![pointer.id.clone()];
+        child.status = PointerStatus::Complete;
+        workflow.execution_pointers.push(child);
+
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+        let result = step.run(&ctx).await.unwrap();
+        assert!(result.proceed);
+    }
+
+    #[tokio::test]
+    async fn children_incomplete_persists() {
+        let mut step = SequenceStep;
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+
+        let mut workflow = default_workflow();
+        let mut child = ExecutionPointer::new(1);
+        child.scope = vec![pointer.id.clone()];
+        child.status = PointerStatus::Running;
+        workflow.execution_pointers.push(child);
+
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+        let result = step.run(&ctx).await.unwrap();
+        assert!(!result.proceed);
+        assert_eq!(result.persistence_data, Some(json!({"children_active": true})));
+    }
+
+    #[tokio::test]
+    async fn no_children_in_scope_proceeds() {
+        let mut step = SequenceStep;
+        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();
+        // No children in scope means is_branch_complete returns true (vacuously).
+        assert!(result.proceed);
+    }
+}