feat: add superfig paper-figure toolkit

Standalone LaTeX/TikZ skill for non-tensor paper figures: node/edge
macros, lint-on-warning build, golden examples, and negative fixtures.

examples/state-flow.tex

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+% superfig golden example 3 -- a state/time flow.
+% One decode step: previous KV plus a new token produce the next state
+% and the emitted token. The callout is an aside on the finished state band.
+%   ../scripts/build.sh state-flow.tex
+\documentclass[border=10pt]{standalone}
+\usepackage[cjk]{superfig}
+
+\sfsetrole{state}{sfTeal}
+\sfsetrole{step}{sfOrange}
+\sfsetrole{tok}{sfViolet}
+
+\begin{document}
+\begin{tikzpicture}
+
+\sfstage{S}{解码一步：旧状态 + 新 token}
+\sfvgap{26mm}
+\sfrow{R1}{14mm}
+  \sfnode[role=state]{kv}{KV$_t$}{16mm}{11mm}
+  \sfconn{c1}{读}
+  \sfnode[role=step]{dec}{解码}{16mm}{11mm}
+  \sfconn{c2}{追加}
+  \sfnode[role=state]{kv2}{KV$_{t+1}$}{18mm}{11mm}
+\sfrowend
+\sflane{R1}
+\sfcaption{kv}{KV$_t$}{已缓存键值}
+\sfcaption{dec}{一步}{读 KV，写新列}
+\sfcaption{kv2}{KV$_{t+1}$}{状态推进}
+\sfcallout{N1}{38mm}{R1}{为何保留 KV}{%
+  下一步只读已有列、只追加 $x_t$ 这一列。重算前缀是另一条路径，这张图不画。}
+
+% Tokens enter from above so the caption lane below the state row stays clear.
+\sfnode[role=tok, at={($(dec.north west)+(-4mm,14mm)$)}]{xt}{$x_t$}{14mm}{10mm}
+\sfnode[role=tok, at={($(dec.north east)+(4mm,14mm)$)}]{yt}{$y_t$}{14mm}{10mm}
+\sfarrow{xt.south}{dec.north}
+\sfarrow{dec.north}{yt.south}
+
+\sfbbox{all}
+\sftopformula{F}{%
+  $(\mathrm{KV}_{t+1},\, y_t) = \mathrm{Decode}(\mathrm{KV}_t,\, x_t)$}
+\sfmeaningbox{mb}{108mm}{all}
+  {自回归一步：状态沿时间推进}
+  {缓存 KV、解码算子、本步 token}
+  {解码读 $\mathrm{KV}_t$ 与 $x_t$，写出 $y_t$，并把新列追加为 $\mathrm{KV}_{t+1}$}
+
+\end{tikzpicture}
+\end{document}