Pure formatting pass from `cargo fmt --all`. No logic changes. Separating
this out so the 1.9 release feature commits that follow show only their
intentional edits.
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.