3-colour DFS cycle detection. Returns one cycle witness (a list of node ids with the closing id repeated, e.g. `[a; b; c; a]`) if any cycle is present, otherwise `None`.

Kahn topological sort with a deterministic ascending-`NodeId` tie-break. `Ok order` lists every node in a valid run order; `Error remaining` lists the nodes that could not be ordered because they participate in (or depend on) a cycle. Edges that reference unknown nodes are ignored.