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/models/execution_error.rs

@@ -0,0 +1,48 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ExecutionError {
+    pub error_time: DateTime<Utc>,
+    pub workflow_id: String,
+    pub execution_pointer_id: String,
+    pub message: String,
+}
+
+impl ExecutionError {
+    pub fn new(
+        workflow_id: impl Into<String>,
+        execution_pointer_id: impl Into<String>,
+        message: impl Into<String>,
+    ) -> Self {
+        Self {
+            error_time: Utc::now(),
+            workflow_id: workflow_id.into(),
+            execution_pointer_id: execution_pointer_id.into(),
+            message: message.into(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn new_error_captures_fields() {
+        let err = ExecutionError::new("wf-1", "ptr-1", "something went wrong");
+        assert_eq!(err.workflow_id, "wf-1");
+        assert_eq!(err.execution_pointer_id, "ptr-1");
+        assert_eq!(err.message, "something went wrong");
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let err = ExecutionError::new("wf-1", "ptr-1", "fail");
+        let json = serde_json::to_string(&err).unwrap();
+        let deserialized: ExecutionError = serde_json::from_str(&json).unwrap();
+        assert_eq!(err.workflow_id, deserialized.workflow_id);
+        assert_eq!(err.message, deserialized.message);
+    }
+}