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/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,
 }