Redraw n=21 witness figures as crossing-free planar graphs

Replace the radial (crossing-heavy) figure with two crossing-free planar
drawings (networkx planar_layout / Chrobak-Payne):
  fig:n21-elgs  -- the six witness Even Level Graphs, parity-coloured, with
                   the bridge-switch-flipped edges dashed red;
  fig:n21-duals -- the six resulting duals, with the introduced bridge edges
                   solid green.
ELG and dual are drawn with independent planar layouts so neither has any
edge crossing (a flip diagonal would otherwise cross other edges when its
quadrilateral is non-convex, which happens for duals 0 and 3). Drop forced
equal aspect so panels fill and labels separate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

papers/even_level_graph_generators/paper.tex

@@ -428,15 +428,28 @@ witnesses are step-verified.}
 
 \begin{figure}[ht]
 \centering
-\includegraphics[width=\textwidth]{figures/n21_witnesses.png}
+\includegraphics[width=\textwidth]{figures/n21_elgs.png}
 \caption{The witness Even Level Graph for each of the six Holton--McKay
-duals, drawn radially by level (source at the centre, level-$k$ vertices on
-ring $k$) and coloured by parity (blue even, orange odd). Dashed red edges
-are the same-parity edges flipped by the bridge switches; solid green edges
-are the bridge edges they introduce; applying the switches turns each Even
-Level Graph into the corresponding dual. Duals $1$ and $2$ are Even Level
-Graphs outright, so no switch is shown.}
-\label{fig:n21-witnesses}
+duals, drawn as a crossing-free planar graph and coloured by parity (blue
+even, orange odd, with respect to the fixed level-parity labelling). The
+dashed red edges are the same-parity edges that the bridge switches flip;
+flipping them yields the corresponding dual in
+Figure~\ref{fig:n21-duals}. Duals $1$ and $2$ are Even Level Graphs
+outright, so no edge is flipped.}
+\label{fig:n21-elgs}
+\end{figure}
+
+\begin{figure}[ht]
+\centering
+\includegraphics[width=\textwidth]{figures/n21_duals.png}
+\caption{The six Holton--McKay duals, drawn as crossing-free planar graphs
+with the same parity colouring. The solid green edges are the bridge edges
+introduced by the switches from the Even Level Graphs of
+Figure~\ref{fig:n21-elgs}. Each green edge is a bridge of its parity
+subgraph, so no new cycle -- and in particular no odd cycle -- is created;
+duals $1$ and $2$ coincide with their Even Level Graphs and have no added
+edge.}
+\label{fig:n21-duals}
 \end{figure}
 
 \begin{thebibliography}{9}