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/step.rs

@@ -0,0 +1,35 @@
+use async_trait::async_trait;
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+
+use crate::models::{ExecutionPointer, ExecutionResult, WorkflowInstance, WorkflowStep};
+
+/// Marker trait for all data types that flow between workflow steps.
+/// Anything that is serializable and deserializable qualifies.
+pub trait WorkflowData: Serialize + DeserializeOwned + Send + Sync + Clone + 'static {}
+
+/// Blanket implementation: any type satisfying the bounds is WorkflowData.
+impl<T> WorkflowData for T where T: Serialize + DeserializeOwned + Send + Sync + Clone + 'static {}
+
+/// Context available to a step during execution.
+#[derive(Debug)]
+pub struct StepExecutionContext<'a> {
+    /// The current item when iterating (ForEach).
+    pub item: Option<&'a serde_json::Value>,
+    /// The current execution pointer.
+    pub execution_pointer: &'a ExecutionPointer,
+    /// Persistence data from a previous execution of this step.
+    pub persistence_data: Option<&'a serde_json::Value>,
+    /// The step definition.
+    pub step: &'a WorkflowStep,
+    /// The running workflow instance.
+    pub workflow: &'a WorkflowInstance,
+    /// Cancellation token.
+    pub cancellation_token: tokio_util::sync::CancellationToken,
+}
+
+/// The core unit of work in a workflow. Each step implements this trait.
+#[async_trait]
+pub trait StepBody: Send + Sync {
+    async fn run(&mut self, context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult>;
+}