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/status.rs

@@ -0,0 +1,73 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum WorkflowStatus {
+    #[default]
+    Runnable,
+    Suspended,
+    Complete,
+    Terminated,
+}
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum PointerStatus {
+    #[default]
+    Pending,
+    Running,
+    Complete,
+    Sleeping,
+    WaitingForEvent,
+    Failed,
+    Compensated,
+    Cancelled,
+    PendingPredecessor,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn workflow_status_default_is_runnable() {
+        assert_eq!(WorkflowStatus::default(), WorkflowStatus::Runnable);
+    }
+
+    #[test]
+    fn pointer_status_default_is_pending() {
+        assert_eq!(PointerStatus::default(), PointerStatus::Pending);
+    }
+
+    #[test]
+    fn workflow_status_serde_round_trip() {
+        for status in [
+            WorkflowStatus::Runnable,
+            WorkflowStatus::Suspended,
+            WorkflowStatus::Complete,
+            WorkflowStatus::Terminated,
+        ] {
+            let json = serde_json::to_string(&status).unwrap();
+            let deserialized: WorkflowStatus = serde_json::from_str(&json).unwrap();
+            assert_eq!(status, deserialized);
+        }
+    }
+
+    #[test]
+    fn pointer_status_serde_round_trip() {
+        for status in [
+            PointerStatus::Pending,
+            PointerStatus::Running,
+            PointerStatus::Complete,
+            PointerStatus::Sleeping,
+            PointerStatus::WaitingForEvent,
+            PointerStatus::Failed,
+            PointerStatus::Compensated,
+            PointerStatus::Cancelled,
+            PointerStatus::PendingPredecessor,
+        ] {
+            let json = serde_json::to_string(&status).unwrap();
+            let deserialized: PointerStatus = serde_json::from_str(&json).unwrap();
+            assert_eq!(status, deserialized);
+        }
+    }
+}