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/lifecycle.rs

@@ -0,0 +1,9 @@
+use async_trait::async_trait;
+
+use crate::models::LifecycleEvent;
+
+/// Publishes lifecycle events for workflow state transitions.
+#[async_trait]
+pub trait LifecyclePublisher: Send + Sync {
+    async fn publish(&self, event: LifecycleEvent) -> crate::Result<()>;
+}