Hierarchical layout (Sugiyama method)

Directed graphs need a hierarchical/layered layout, which is what Graphviz's `dot` engine produces. This is the Sugiyama method, consisting of four sub-steps.

First, cycle removal via DFS to identify and reverse back edges, storing a `reversed` flag so rendering can restore the original direction. Second, layer assignment using longest-path layering, which assigns each node to a rank. Edges spanning multiple layers get virtual (dummy) nodes inserted so every edge spans exactly one layer; these become bend points in the final rendering. Third, crossing minimization using the barycenter heuristic with alternating sweeps (~24 passes) to reduce edge crossings between adjacent layers. Fourth, coordinate assignment using a simplified Brandes-Kopf approach, spacing nodes within layers by `nodesep` (default 72pt) and layers by `ranksep` (default 36pt). The `rankdir` attribute (TB/BT/LR/RL) is handled by swapping axis semantics.

Cluster handling requires constraining cluster nodes to contiguous positions within their layers, then computing cluster bounding boxes after coordinate assignment. Special cases include self-loops (excluded from layering, rendered as curved paths), multi-edges, disconnected components (laid out independently then composed), and the `rank` attribute for same/min/max/source/sink constraints.

References

• Layered graph drawing (Wikipedia)
• Sugiyama method (not yet a page, see Layered graph drawing)
• A Technique for Drawing Directed Graphs (Gansner et al., 1993)
• Fast and Simple Horizontal Coordinate Assignment (Brandes & Kopf, 2001)
• Topological sorting (Wikipedia)
• Network simplex algorithm (Wikipedia)

No implementation files or tests are named. Start by reviewing the linked Graphviz and layered graph drawing references, then map the four required Sugiyama stages and the listed cluster, loop, multi-edge, component, and rank constraints onto the renderer. Done means hierarchical layouts support those stages, rankdir, spacing, and the stated special cases.