feat(wfe-core): add models, traits, and error types

Core domain models: WorkflowInstance, ExecutionPointer, WorkflowDefinition,
WorkflowStep, Event, EventSubscription, ScheduledCommand, ExecutionError,
LifecycleEvent, PollEndpointConfig. All serde-serializable.

Provider traits: PersistenceProvider (composite of WorkflowRepository,
EventRepository, SubscriptionRepository, ScheduledCommandRepository),
DistributedLockProvider, QueueProvider, SearchIndex, LifecyclePublisher,
WorkflowMiddleware, StepMiddleware, WorkflowRegistry.

StepBody trait with StepExecutionContext for workflow step implementations.
WorkflowData marker trait (blanket impl for Serialize + DeserializeOwned).

wfe-core/src/traits/queue.rs

@@ -0,0 +1,13 @@
+use async_trait::async_trait;
+
+use crate::models::QueueType;
+
+/// Queue provider for distributing workflow execution across workers.
+#[async_trait]
+pub trait QueueProvider: Send + Sync {
+    async fn queue_work(&self, id: &str, queue: QueueType) -> crate::Result<()>;
+    async fn dequeue_work(&self, queue: QueueType) -> crate::Result<Option<String>>;
+    fn is_dequeue_blocking(&self) -> bool;
+    async fn start(&self) -> crate::Result<()>;
+    async fn stop(&self) -> crate::Result<()>;
+}