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-core/src/executor/workflow_executor.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
 use chrono::Utc;
-use tracing::{debug, error, warn};
+use tracing::{debug, error, info, warn};
 
 use super::error_handler;
 use super::result_processor;
@@ -106,6 +106,12 @@ impl WorkflowExecutor {
         let mut execution_errors = Vec::new();
 
         // 3. Find runnable execution pointers.
+        info!(
+            workflow_id,
+            definition_id = %workflow.workflow_definition_id,
+            pointers = workflow.execution_pointers.len(),
+            "Executing workflow"
+        );
         let runnable_indices: Vec<usize> = workflow
             .execution_pointers
             .iter()
@@ -125,6 +131,14 @@ impl WorkflowExecutor {
                 .find(|s| s.id == step_id)
                 .ok_or(WfeError::StepNotFound(step_id))?;
 
+            info!(
+                workflow_id,
+                step_id,
+                step_type = %step.step_type,
+                step_name = step.name.as_deref().unwrap_or("(unnamed)"),
+                "Running step"
+            );
+
             // b. Resolve the step body.
             let mut step_body = step_registry
                 .resolve(&step.step_type)
@@ -156,6 +170,15 @@ impl WorkflowExecutor {
             // Now we can mutate again since context is dropped.
             match step_result {
                 Ok(result) => {
+                    info!(
+                        workflow_id,
+                        step_id,
+                        proceed = result.proceed,
+                        has_sleep = result.sleep_for.is_some(),
+                        has_event = result.event_name.is_some(),
+                        has_branches = result.branch_values.is_some(),
+                        "Step completed"
+                    );
                     // e. Process the ExecutionResult.
                     // Extract workflow_id before mutable borrow.
                     let wf_id = workflow.id.clone();
@@ -225,6 +248,7 @@ impl WorkflowExecutor {
             });
 
         if all_done && workflow.status == WorkflowStatus::Runnable {
+            info!(workflow_id, "All pointers complete, workflow finished");
             workflow.status = WorkflowStatus::Complete;
             workflow.complete_time = Some(Utc::now());
         }

wfe-core/src/models/poll_config.rs

@@ -3,8 +3,9 @@ use std::time::Duration;
 
 use serde::{Deserialize, Serialize};
 
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
 pub enum HttpMethod {
+    #[default]
     Get,
     Post,
     Put,
@@ -25,7 +26,13 @@ pub enum PollCondition {
     BodyContains(String),
 }
 
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+impl Default for PollCondition {
+    fn default() -> Self {
+        Self::StatusCode(200)
+    }
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
 pub struct PollEndpointConfig {
     /// URL template. Supports `{placeholder}` interpolation from workflow data.
     pub url: String,

wfe-core/src/primitives/decide.rs

@@ -4,6 +4,7 @@ use crate::models::ExecutionResult;
 use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A decision step that returns an outcome value for routing.
+#[derive(Default)]
 pub struct DecideStep {
     pub expression_value: serde_json::Value,
 }

wfe-core/src/primitives/end_step.rs

@@ -4,6 +4,7 @@ use crate::models::ExecutionResult;
 use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A no-op marker step indicating the end of a workflow branch.
+#[derive(Default)]
 pub struct EndStep;
 
 #[async_trait]

wfe-core/src/primitives/if_step.rs

@@ -5,6 +5,7 @@ use crate::models::ExecutionResult;
 use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A conditional step that branches execution based on a boolean condition.
+#[derive(Default)]
 pub struct IfStep {
     pub condition: bool,
 }

wfe-core/src/primitives/poll_endpoint.rs

@@ -6,6 +6,7 @@ use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A step that polls an external HTTP endpoint until a condition is met.
 /// The actual HTTP polling is handled by the executor, not this step.
+#[derive(Default)]
 pub struct PollEndpointStep {
     pub config: PollEndpointConfig,
 }

wfe-core/src/primitives/recur.rs

@@ -8,6 +8,7 @@ use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A step that repeatedly schedules child execution at an interval
 /// until a stop condition is met.
+#[derive(Default)]
 pub struct RecurStep {
     pub interval: Duration,
     pub stop_condition: bool,

wfe-core/src/primitives/schedule.rs

@@ -7,6 +7,7 @@ use crate::models::ExecutionResult;
 use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A step that schedules child execution after a delay.
+#[derive(Default)]
 pub struct ScheduleStep {
     pub interval: Duration,
 }

wfe-core/src/primitives/sequence.rs

@@ -6,6 +6,7 @@ use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A container step that executes its children sequentially.
 /// Completes when all children have finished.
+#[derive(Default)]
 pub struct SequenceStep;
 
 #[async_trait]

wfe-core/src/primitives/while_step.rs

@@ -5,6 +5,7 @@ use crate::models::ExecutionResult;
 use crate::traits::step::{StepBody, StepExecutionContext};
 
 /// A looping step that repeats its children while a condition is true.
+#[derive(Default)]
 pub struct WhileStep {
     pub condition: bool,
 }