Add diagrams of the six witness Even Level Graphs and their bridge switches

Figure fig:n21-witnesses: each of the six Holton-McKay duals drawn as its
witness Even Level Graph in a radial-by-level layout (source centre,
level-k vertices on ring k), coloured by parity. Dashed red edges are the
flipped same-parity edges and solid green edges the introduced bridges;
applying the switches yields the dual. Duals 1,2 are ELGs outright.

draw_witnesses.py generates the combined 2x3 figure and per-dual PNGs from
the verified witness JSONs.

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

papers/even_level_graph_generators/paper.tex

@@ -426,6 +426,19 @@ witnesses are step-verified.}
 \label{tab:n21}
 \end{table}
 
+\begin{figure}[ht]
+\centering
+\includegraphics[width=\textwidth]{figures/n21_witnesses.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}
+\end{figure}
+
 \begin{thebibliography}{9}
 
 \bibitem{holton-mckay}