Add per-dual summary table for the six Holton-McKay duals at n=21

Table tab:n21 records, for each of the six duals: not an intertwining tree;
Even Level Graph source (duals 1,2 only); and bridge-switch path length to
an ELG (0,0 for the two ELG-outright cases; 3,1,2,4 for the rest). All six
are bridge-derived; all witnesses step-verified.

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

papers/even_level_graph_generators/paper.tex

@@ -405,7 +405,26 @@ classes: two are Even Level Graphs outright, and the other four are
 bridge-derived level graphs. The bridge-switch restriction is what made
 the search tractable -- it both shrinks the orbit and guarantees
 validity, so any Even Level Graph located in a backward orbit is an
-immediate witness.
+immediate witness. Table~\ref{tab:n21} records the outcome for each dual.
+
+\begin{table}[ht]
+\centering
+\begin{tabular}{cccc}
+dual & intertwining tree & Even Level Graph source & bridge switches to ELG \\\hline
+$0$ & no & --       & $3$ \\
+$1$ & no & $10$     & $0$ \\
+$2$ & no & $9$      & $0$ \\
+$3$ & no & --       & $1$ \\
+$4$ & no & --       & $2$ \\
+$5$ & no & --       & $4$ \\
+\end{tabular}
+\caption{The six Holton--McKay duals at $n = 21$, the first triangulations
+that are not intertwining trees. Each is a bridge-derived level graph:
+duals $1$ and $2$ are Even Level Graphs outright (zero switches), and the
+remaining four reach an Even Level Graph in $1$--$4$ bridge switches. All
+witnesses are step-verified.}
+\label{tab:n21}
+\end{table}
 
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