feat: add superderive stepwise derivation figures

A third sibling toolkit: rewrite steps, justification rails,
cancel/substitute highlights, and lint-on-warning builds.

references/antipatterns.md

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+# Antipatterns
+
+- Drawing a rewrite as `\sfnode` boxes.
+- A new hue per step.
+- Two illegal/legal pairs in one figure.
+- Formula placed before the last row so it centers on an unknown width.

references/api.md

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+# API
+
+`\usepackage[cjk]{superderive}` or `[en]`.
+
+Layout: `\sdstage{n}{text}` `\sdrow[height]{name}` … `\sdrowend` `\sdbbox{all}` `\sdtopformula{F}{math}`.
+
+Call `\sdtopformula` after the last `\sdrowend`.

references/checklist.md

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+# Checklist
+
+- Each step is one rewrite. Reasons sit on the rail, not on the shaft.
+- Cancel/subst encode the actual cancelled or replaced term.
+- At most one `\sdcol`. At most four active hues plus gray.
+- No raw `\draw`. Formula after the last row.
+- Clean `build.sh` (missing glyphs, overfull, package warnings).

references/fallback.md

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+# Fallback
+
+`preflight.sh` 0/1/2 matches superfig. Exit 2: no XeLaTeX. Exit 1: no CJK or no pdftocairo.

references/grammar.md

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+# Grammar
+
+| Fact | Macro |
+|---|---|
+| one legal rewrite | `\sdstep[role=rewrite]{name}{lhs}{rhs}` |
+| why it is legal | `\sdreason{name}{text}` |
+| a cancelled term | `\sdcancel{name}{math}` |
+| substitution | `\sdsubst{name}{from}{to}` |
+| focus box | `\sdbox[role=keep]{name}{math}` |
+| illegal vs legal | `\sdcol{name}{bad}{good}` (one per figure) |
+| bottom | `\sdmeaningbox` (idea / rewrite / caveat) |
+
+Built-in roles: `keep` `rewrite` `cancel` `intro` `warn` `neutral`.

references/layout.md

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+# Layout
+
+Place by cursor inside `\sdrow` … `\sdrowend`. Never hand-tune a neighbour with a magic offset. If a reason does not fit, move the step to the next row.

references/style.md

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+# Style
+
+Palette `sdTeal #4F8FA5` `sdOrange #EE995B` `sdCoral #C95B5B` `sdViolet #8A74B5` `sdGray #85898F`. One hue per role. ≤4 active hues plus gray. Lightness `30/55/80`.