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/checklist.md

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+# Delivery checklist
+
+A clean build only proves TeX and package invariants. Inspect the figure before delivery.
+
+## Semantics
+
+- [ ] Every node is one object with one role.
+- [ ] Every edge is data flow, control flow, dependency, or causality.
+- [ ] Every group names a real composite, binds at least two members, and all members belong to it.
+- [ ] At most one `\sfcallout`, hanging off a finished band, not off a single node.
+- [ ] Captions explain objects or relationships, not decoration.
+
+## Layout
+
+- [ ] No node covers another node, caption, edge label, or group border.
+- [ ] Flow rows start on the same left rail.
+- [ ] Edge labels sit on their own edge and do not cover unrelated objects.
+- [ ] Fixed coordinates are on a coarse grid, not tuned against one label width.
+- [ ] Captions in a row share one baseline.
+- [ ] Meaning box is one column, at most three rows, no overflow.
+
+## Style
+
+- [ ] One role keeps one hue across the whole figure.
+- [ ] At most four active hue families plus gray.
+- [ ] Three visibly separated lightness levels where objects need hierarchy; no uniform pastel.
+- [ ] Borders are neutral; group outlines carry their members' role hue.
+- [ ] No saturated primaries, shadows, banners, or decorative cards.
+
+## Thumbnail
+
+- [ ] Open `*-thumb.png` (360 px). The main claim is still legible.
+- [ ] Hues remain distinct and muted; text has not collapsed into gray mush.
+
+## Delivery
+
+- [ ] PNG preview shown.
+- [ ] One-paragraph mechanism explanation in prose.
+- [ ] `.tex` source and PDF/SVG linked.
+- [ ] Any degraded path or inferred relationship stated explicitly.