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/error.rs

@@ -0,0 +1,39 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum WfeError {
+    #[error("Workflow not found: {0}")]
+    WorkflowNotFound(String),
+
+    #[error("Workflow definition not found: {id} v{version}")]
+    DefinitionNotFound { id: String, version: u32 },
+
+    #[error("Event not found: {0}")]
+    EventNotFound(String),
+
+    #[error("Subscription not found: {0}")]
+    SubscriptionNotFound(String),
+
+    #[error("Step not found: {0}")]
+    StepNotFound(usize),
+
+    #[error("Lock acquisition failed for: {0}")]
+    LockFailed(String),
+
+    #[error("Persistence error: {0}")]
+    Persistence(String),
+
+    #[error("Serialization error: {0}")]
+    Serialization(#[from] serde_json::Error),
+
+    #[error("Step execution error: {0}")]
+    StepExecution(String),
+
+    #[error("Workflow cancelled")]
+    Cancelled,
+
+    #[error(transparent)]
+    Other(#[from] Box<dyn std::error::Error + Send + Sync>),
+}
+
+pub type Result<T> = std::result::Result<T, WfeError>;