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.

references/style.md

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+# House style
+
+The package ships these defaults; this file explains what to preserve and what must still
+be decided.
+
+## Canvas
+
+White background, natural `standalone` crop. No forced 16:9. No title by default. The top
+holds at most two compact claim/formula lines.
+
+Each stage is one horizontal row with a shared baseline. Use the fewest stages that preserve
+the primary claim. No unrelated branches, no dashboard panels.
+
+## Type hierarchy
+
+| element | size | macro / style |
+|---|---|---|
+| top formula/claim | `\large` | `\sftopformula` |
+| stage label | `\small\bfseries`, muted | `\sfstage` |
+| node label | `\small` | `\sfnode` |
+| caption symbol | `\small` | `\sfcaption` |
+| caption detail | `\scriptsize`, muted | `\sfcaption` |
+| edge label | `\scriptsize`, muted | `\sfconn` / `\sfarrowlabel` |
+| meaning box | `\small` | `\sfmeaningbox` |
+| optional signature | `\scriptsize`, low contrast | `\sfsignature` |
+
+Never shrink below this hierarchy to make something fit. Move it to another row instead.
+
+## Color
+
+Palette: `sfTeal #4F8FA5`, `sfOrange #EE995B`, `sfCoral #C95B5B`, `sfViolet #8A74B5`,
+`sfGray #85898F`. Muted, mid-chroma, paper-like. Do not add saturated primaries.
+
+- One semantic color per role, held everywhere: `\sfsetrole{input}{sfTeal}`.
+- Drawing macros take a **role**, never a color.
+- At most four active hue families per figure, plus gray.
+- Contrast comes from lightness separation, not saturation. Use `level=1/2/3` for
+  `role!30 / role!55 / role!80`; do not use uniform pastel fills.
+- If sign matters, use hue for sign and intensity for magnitude.
+
+## Edges and nodes
+
+- Nodes are rounded rectangles with thin neutral borders; no saturated colored outlines.
+- Group outlines use `role!65` because the outline itself names an object.
+- Edges are thin neutral arrows on the background layer; text stays on the foreground layer.
+- Keep all text in LaTeX or CJK text, never rasterized labels.
+
+## Never
+
+Metric insets not present in the primary claim. Decorative pills, banners, shadows,
+repeated separators, extra explanatory cards beyond one `\sfcallout`. Raw TikZ `\draw` /
+`\fill` / `\path` unless the lint directive allows it.