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.

tests/flow.tex

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+% Cursor extras: gap, leftover rail, a tracked absolute node.
+\documentclass[border=8pt]{standalone}
+\usepackage[en]{superfig}
+
+\sfsetrole{x}{sfTeal}
+\sfsetrole{y}{sfViolet}
+
+\begin{document}
+\begin{tikzpicture}
+\sfleftrail{4mm}
+\sfrow{R1}{12mm}
+  \sfnode[role=x]{A}{$A$}{12mm}{10mm}
+  \sfgap{8mm}
+  \sfnode[role=y]{B}{$B$}{12mm}{10mm}
+\sfrowend
+\sfnode[role=x, at={($(B.south)+(0,-14mm)$)}]{C}{$C$}{12mm}{10mm}
+\sfarrow{B.south}{C.north}
+\sflane{R1}
+\sfcaption{A}{$A$}{rail}
+\sfnolane
+\sfcaptiontop{B}{after gap}
+\sfcaption{C}{$C$}{absolute, tracked}
+\sfbbox{all}
+\sftopformula{F}{$C \leftarrow B$}
+\end{tikzpicture}
+\end{document}