Add Even Level Graph Generators paper + extend Level Switching reachability

- New paper papers/even_level_graph_generators/: defines Even Level
  Graph (every level cycle even), derived level graphs, intertwining
  trees, and the disjunction conjecture (every maximal planar graph is
  a derived level graph or intertwining tree). Empirically tested
  through n=11: every iso class is at least an intertwining tree, so
  the disjunction holds trivially in this range. The intertwining tree
  disjunct fails at the Tutte graph dual (n=25), so the disjunction
  becomes non-trivial past some unknown threshold.

- Level Switching paper: adds Section 4 (Reachability via edge
  switches) with the two-step argument (Sleator-Tarjan-Thurston for
  Case 1; face-merges for Case 2) and Theorem 4.1 (O(n) edge switches
  suffice to reach all-depth-0).

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

papers/even_level_graph_generators/paper.aux

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+\newlabel{fig:levels}{{1}{1}{BFS levels from the degree-$3$ vertex source $S = \{4\}$. The source is level $0$, its three neighbours are level $1$, and the remaining vertices are level $2$. Colour encodes the level}{figure.1}{}}
+\@writefile{lof}{\contentsline {figure}{\numberline {2}{\ignorespaces A level cycle in the triangulation of Figure\nonbreakingspace \ref  {fig:levels}. The triangle $1\!-\!2\!-\!3$ is a simple cycle whose three vertices all lie at level $1$, so it is a level cycle at level $1$.}}{2}{figure.2}\protected@file@percent }
+\newlabel{fig:level-cycle}{{2}{2}{A level cycle in the triangulation of Figure~\ref {fig:levels}. The triangle $1\!-\!2\!-\!3$ is a simple cycle whose three vertices all lie at level $1$, so it is a level cycle at level $1$}{figure.2}{}}
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+\newlabel{fig:parity-subgraph}{{4}{3}{Parity subgraphs of $G' = T$ with respect to the level structure of Figure~\ref {fig:levels} (here we take $G = G' = T$). Left: $T$ with vertices coloured by $\ell _G \bmod 2$ (blue $=$ even, orange $=$ odd). Middle: the even parity subgraph $E_{G,S}(G')$, induced on $\{0, 4, 5, 6\}$; only edges with both endpoints even appear. Right: the odd parity subgraph $O_{G,S}(G')$, induced on $\{1, 2, 3\}$; the highlighted triangle shows that $O_{G,S}(G')$ is not bipartite for this choice of $G'$}{figure.4}{}}
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