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/grammar.md

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+# Visual grammar
+
+Choose the smallest grammar that exposes the paper's mechanism. Adding a second grammar
+must add information.
+
+| Fact to expose | Grammar |
+|---|---|
+| one semantic object | `\sfnode` |
+| linear sequence inside one band | `\sfstage` + `\sfrow` … `\sfrowend` |
+| an edge between two objects | `\sfconn` for flow, `\sfarrow` for fixed topology |
+| an edge label | `\sfconn{name}{label}` or `\sfarrowlabel` |
+| a real composite (module, subsystem, shared owner) | `\sfgroup` |
+| an operator inside a flow row | `\sfop` |
+| a side note about a finished band | `\sfcallout` |
+| the bottom explanation | `\sfmeaningbox` |
+
+## Node semantics
+
+One block is one semantic object. If you need two verbs in one block, split it.
+
+- Label is the object's name or role, not a sentence.
+- Fill color encodes a role, not importance.
+- Width/height may differ to reflect a visual hierarchy, but do not use size to invent
+  quantitative meaning unless the figure says so.
+
+## Edge semantics
+
+Every arrow must be one of:
+
+- **data flow** — the output of A becomes the input of B.
+- **control flow** — A decides whether or when B runs.
+- **dependency** — B needs A to exist or to have run.
+- **causality** — A causes B.
+
+A decorative arrow is an error. If an edge does not carry one of those meanings, remove it
+or replace it with a grouping/caption.
+
+## Group semantics
+
+`\sfgroup` names a composite object. The members inside must actually belong to that
+composite in the paper. Do not draw an outline around nearby nodes merely because it looks
+balanced. The fit list is explicit: `{(node1)(node2)}`.
+
+## Captions and meaning box
+
+- `\sfcaption{name}{symbol}{detail}`: one short symbol line plus one muted detail line.
+- `\sfmeaningbox`: at most three rows — idea, objects, mechanism. Prefer removing content
+  over shrinking type or adding columns.