@spec assign_layers(Boxart.Graph.t()) :: %{required(String.t()) => non_neg_integer()}

Assign each node to a layer based on longest path from a root.

Back-edges (edges that would create cycles) are excluded from layer computation to prevent infinite loops and excessive layers.

@spec compute_gap_expansions(Boxart.Graph.t(), [[String.t()]]) :: %{
  required(non_neg_integer()) => non_neg_integer()
}

Compute extra grid cells needed between adjacent layers for crossing edges.

Returns a map of gap index (between layer i and i+1) to the number of extra grid cells to insert.

@spec order_layers(Boxart.Graph.t(), %{required(String.t()) => non_neg_integer()}) ::
  [[String.t()]]

Order nodes within each layer using barycenter heuristic for crossing minimization.

@spec separate_subgraph_layers(Boxart.Graph.t(), %{
  required(String.t()) => non_neg_integer()
}) :: %{
  required(String.t()) => non_neg_integer()
}

Fix overlapping subgraph layer ranges.

When cross-boundary edges cause nodes from different subgraphs to land on the same layer, the subgraph boxes overlap visually. Reassigns layers so each subgraph occupies a contiguous, non-overlapping range.