Confirm duals 1,2 are Even Level Graphs outright; archive all six witnesses

Tested duals 1 and 2: both are Even Level Graphs directly (dual 1 for
source 10, dual 2 for source 9), so bridge-derived with a zero-length
switch sequence. All six Holton-McKay duals are confirmed non-intertwining
(consistent with the dual-Hamiltonian theorem, since all six HM graphs are
non-Hamiltonian) and all six are bridge-derived. Saved witness files
dual_1.json, dual_2.json (0 switches) to complete the archive for all six.
Updated the n=21 subsection accordingly.

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

papers/even_level_graph_generators/paper.tex

@@ -396,8 +396,9 @@ partitions of the same triangulations have orbits exceeding $10^6$;
 finding one good partition suffices. Each witness is in fact only a
 \emph{handful} of bridge switches from its dual: the explicit Even Level
 Graph, parity labelling, and bridge-switch sequence are recorded for all
-four, with path lengths $3, 1, 2, 4$ respectively, and each step has been
-verified to be a valid bridge switch.
+six -- path lengths $3,1,2,4$ for these four and $0$ for the two that are
+Even Level Graphs outright -- and each step has been verified to be a
+valid bridge switch.
 \end{itemize}
 Thus at $n = 21$ the disjunction is confirmed for all six critical iso
 classes: two are Even Level Graphs outright, and the other four are