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/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]