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/layout.md

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+# Layout rules
+
+## Cursor flow
+
+Inside `\sfrow` … `\sfrowend`, objects are placed from left to right by the cursor.
+The first object starts on the left rail; every later object reserves the standing gutter.
+
+- Declare the band height once in `\sfrow`. An object that overflows it is a build warning.
+- Use `\sfconn` for a labelled edge in the flow; the label reserves its own width.
+- Do not add magic-number `xshift`s between flow objects.
+
+## Fixed topology
+
+Use `\sfnode[at={(x,y)}]{...}` only when the diagram is genuinely non-linear: branches,
+loops, vertical/horizontal stacks, skip edges.
+
+- Keep coordinates on a coarse grid; prefer multiples of `\sfgutter` or explicit anchors.
+- Use `\sfarrow` between placed nodes. Route edges on the background layer.
+- Put edge labels on the edge itself, not floating nearby.
+
+## Captions
+
+Use `\sflane{row}` after `\sfrowend` to give all captions in that row one shared baseline.
+Symbol and detail lines are two reserved lanes; do not place other text between them.
+
+## Groups
+
+A group outline adds inner padding, so place it after its members. It must not cover unrelated
+nodes. Members must be adjacent in the semantic sense; the explicit fit list prevents the
+package from inventing a group around whatever is near. A group must bind at least two
+members.
+
+After a `\sfrow`, `\sfgroup` refits that row so `\sflane{row}` hangs captions below the
+outline. Prefer an empty overlay caption and `\sfcaption{group}{...}{...}`: an overlay at
+the north-west corner lands on skip edges. A connector that should leave the composite
+starts on the group node (`\sfarrow{block.east}{next.west}`), not on a member — otherwise
+the shaft crosses an outline that is not its endpoint.
+
+## Formula
+
+Call `\sftopformula` after the last `\sfrowend`. The line is centered on `\sfbbox`, whose
+width is only known once the bands exist.
+
+## Callouts
+
+`\sfcallout` hangs off a finished `\sfrow` band, one per figure. It is not an object in
+the flow and must not be anchored to a single node.
+
+## Meaning box
+
+The meaning box hangs below the full figure bbox (`\sfbbox`). It is one column, at most
+three rows. If it is too long, remove content; do not widen it past the figure or shrink type.