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/mod.rs

@@ -0,0 +1,65 @@
+pub mod error_behavior;
+pub mod event;
+pub mod execution_error;
+pub mod execution_pointer;
+pub mod execution_result;
+pub mod lifecycle;
+pub mod poll_config;
+pub mod queue_type;
+pub mod scheduled_command;
+pub mod status;
+pub mod workflow_definition;
+pub mod workflow_instance;
+
+pub use error_behavior::ErrorBehavior;
+pub use event::{Event, EventSubscription};
+pub use execution_error::ExecutionError;
+pub use execution_pointer::ExecutionPointer;
+pub use execution_result::ExecutionResult;
+pub use lifecycle::{LifecycleEvent, LifecycleEventType};
+pub use poll_config::{HttpMethod, PollCondition, PollEndpointConfig};
+pub use queue_type::QueueType;
+pub use scheduled_command::{CommandName, ScheduledCommand};
+pub use status::{PointerStatus, WorkflowStatus};
+pub use workflow_definition::{StepOutcome, WorkflowDefinition, WorkflowStep};
+pub use workflow_instance::WorkflowInstance;
+
+/// Serde helper for `Option<Duration>` as milliseconds.
+pub(crate) mod option_duration_millis {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Deserializer, Serializer};
+
+    pub fn serialize<S: Serializer>(
+        duration: &Option<Duration>,
+        serializer: S,
+    ) -> Result<S::Ok, S::Error> {
+        match duration {
+            Some(d) => serializer.serialize_some(&(d.as_millis() as u64)),
+            None => serializer.serialize_none(),
+        }
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(
+        deserializer: D,
+    ) -> Result<Option<Duration>, D::Error> {
+        let millis: Option<u64> = Option::deserialize(deserializer)?;
+        Ok(millis.map(Duration::from_millis))
+    }
+}
+
+/// Serde helper for `Duration` as milliseconds (non-optional).
+pub(crate) mod duration_millis {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Deserializer, Serializer};
+
+    pub fn serialize<S: Serializer>(duration: &Duration, serializer: S) -> Result<S::Ok, S::Error> {
+        serializer.serialize_u64(duration.as_millis() as u64)
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Duration, D::Error> {
+        let millis = u64::deserialize(deserializer)?;
+        Ok(Duration::from_millis(millis))
+    }
+}