feat(wfe-core): add LogSink trait and wire lifecycle publisher into executor

LogSink trait for real-time step output streaming. Added to
StepExecutionContext as optional field (backward compatible).
Threaded through WorkflowExecutor and WorkflowHostBuilder.

Wired LifecyclePublisher.publish() into executor at 5 points:
StepStarted, StepCompleted, Error, Completed, Terminated.
Also added lifecycle events to host start/suspend/resume/terminate.

wfe-core/src/traits/middleware.rs

@@ -69,6 +69,7 @@ mod tests {
             workflow: &instance,
             cancellation_token: tokio_util::sync::CancellationToken::new(),
             host_context: None,
+                log_sink: None,
         };
         mw.pre_step(&ctx).await.unwrap();
     }
@@ -88,6 +89,7 @@ mod tests {
             workflow: &instance,
             cancellation_token: tokio_util::sync::CancellationToken::new(),
             host_context: None,
+                log_sink: None,
         };
         let result = ExecutionResult::next();
         mw.post_step(&ctx, &result).await.unwrap();