The YAML compiler was using SubWorkflowPlaceholderStep that returned
next() immediately. Replaced with real SubWorkflowStep from wfe-core
that starts child workflows and waits for completion events.
Added regression test verifying the compiled factory produces a step
that calls host_context.start_workflow() and returns wait_for_event.
Compiler dispatches type: workflow to SubWorkflowStep. Validation
detects circular workflow references via DFS with coloring. Cross-
workflow reference checking for multi-workflow files. Duplicate
workflow ID detection. 28 edge case tests for validation paths.