feat(wfe-core): add fluent workflow builder API

Owned-self builder pattern (no lifetime parameters). WorkflowBuilder
chains start_with/then/end_workflow to produce WorkflowDefinition.

StepBuilder supports: name, id, on_error, compensate_with, then,
then_fn, wait_for, delay, if_do, while_do, for_each, saga, parallel.

ParallelBuilder for branching with join semantics. InlineStep for
closure-based steps. Step config stored on WorkflowStep.step_config.

wfe-core/src/builder/inline_step.rs

@@ -0,0 +1,30 @@
+use async_trait::async_trait;
+
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+type InlineFn = Box<dyn Fn() -> ExecutionResult + Send + Sync>;
+
+/// A step that wraps an inline closure.
+pub struct InlineStep {
+    body: InlineFn,
+}
+
+impl InlineStep {
+    pub fn new(f: impl Fn() -> ExecutionResult + Send + Sync + 'static) -> Self {
+        Self { body: Box::new(f) }
+    }
+}
+
+impl Default for InlineStep {
+    fn default() -> Self {
+        Self::new(ExecutionResult::next)
+    }
+}
+
+#[async_trait]
+impl StepBody for InlineStep {
+    async fn run(&mut self, _context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        Ok((self.body)())
+    }
+}