Function layout 

pub fn layout(manifest: &Manifest, opts: &LayoutOptions) -> LayoutResult

Computes a layered DAG layout for manifest with the given options.

Nodes are inserted in sorted NodeId order (the fixed tie-break) so equal-rank ordering is stable regardless of input shuffling.

Panics

Debug-asserts that the graph has no cycles (the parse layer already rejects cycles, so this is a defensive invariant check).