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/mod.rs

@@ -0,0 +1,20 @@
+pub mod lifecycle;
+pub mod lock;
+pub mod middleware;
+pub mod persistence;
+pub mod queue;
+pub mod registry;
+pub mod search;
+pub mod step;
+
+pub use lifecycle::LifecyclePublisher;
+pub use lock::DistributedLockProvider;
+pub use middleware::{StepMiddleware, WorkflowMiddleware};
+pub use persistence::{
+    EventRepository, PersistenceProvider, ScheduledCommandRepository, SubscriptionRepository,
+    WorkflowRepository,
+};
+pub use queue::QueueProvider;
+pub use registry::WorkflowRegistry;
+pub use search::{Page, SearchFilter, SearchIndex, WorkflowSearchResult};
+pub use step::{StepBody, StepExecutionContext, WorkflowData};