feat: add executor tracing, auto-register primitives, and Default impls

- Add info!-level tracing to workflow executor: logs each execution
  round, each step run (with type and name), step completion, and
  workflow completion
- WorkflowHost.start() now auto-registers all built-in primitive step
  types so users don't need to register them manually
- Add #[derive(Default)] to all primitive steps and PollEndpointConfig
- Add tracing-subscriber to wfe crate for the pizza example
- Pizza example now shows full step-by-step execution logs

wfe/Cargo.toml

@@ -17,6 +17,8 @@ thiserror = { workspace = true }
 tracing = { workspace = true }
 tokio-util = "0.7"
 
+tracing-subscriber = { workspace = true }
+
 [dev-dependencies]
 wfe-core = { workspace = true, features = ["test-support"] }
 wfe-sqlite = { workspace = true }

wfe/examples/pizza.rs

@@ -444,6 +444,13 @@ fn build_pizza_workflow() -> WorkflowDefinition {
 
 #[tokio::main]
 async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
+    // Set up tracing so we can see the executor's step-by-step logging.
+    tracing_subscriber::fmt()
+        .with_target(false)
+        .with_timer(tracing_subscriber::fmt::time::uptime())
+        .with_env_filter("wfe_core=info,wfe=info")
+        .init();
+
     println!("=== WFE Pizza Workflow Engine Demo ===\n");
 
     // Build the host with in-memory providers (swap for SQLite/Postgres/Valkey in prod)

wfe/src/host.rs

@@ -33,8 +33,27 @@ pub struct WorkflowHost {
 }
 
 impl WorkflowHost {
+    /// Register all built-in primitive step types.
+    async fn register_primitives(&self) {
+        use wfe_core::primitives::*;
+        let mut sr = self.step_registry.write().await;
+        sr.register::<decide::DecideStep>();
+        sr.register::<delay::DelayStep>();
+        sr.register::<end_step::EndStep>();
+        sr.register::<foreach_step::ForEachStep>();
+        sr.register::<if_step::IfStep>();
+        sr.register::<poll_endpoint::PollEndpointStep>();
+        sr.register::<recur::RecurStep>();
+        sr.register::<saga_container::SagaContainerStep>();
+        sr.register::<schedule::ScheduleStep>();
+        sr.register::<sequence::SequenceStep>();
+        sr.register::<wait_for::WaitForStep>();
+        sr.register::<while_step::WhileStep>();
+    }
+
     /// Spawn background polling tasks for processing workflows and events.
     pub async fn start(&self) -> Result<()> {
+        self.register_primitives().await;
         self.queue_provider.start().await?;
         self.lock_provider.start().await?;
         if let Some(ref search) = self.search {