feat: add superderive stepwise derivation figures

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

SKILL.md

@@ -0,0 +1,44 @@
+---
+name: superderive
+description: >-
+  Create or refine stepwise algebraic derivation figures — rewrite
+  steps, justification rails, cancel and substitute highlights.
+  Use when a rewrite must be seen (cancel-visual, subst-visual, or
+  more than four steps), or the user asks for $superderive. Not for
+  architecture (superfig), tensor faces (supertensor), or ordinary
+  note-side align (superpaper v1).
+---
+
+# superderive
+
+Turn one rewrite sequence into one slide-ready figure with three zones:
+
+1. **Top — claim.** The identity being justified.
+2. **Middle — steps.** `\sdstep` / `\sdcancel` / `\sdsubst` / `\sdreason`.
+3. **Bottom — meaning.** Idea, rewrite, caveat.
+
+Draw with the package. Do not hand-roll TikZ unless the linter allows it.
+
+## When to use
+
+- A cancel or substitution that is the claim.
+- More than four rewrite steps that must stay aligned as a picture.
+- An illegal vs legal pair (`\sdcol`, one per figure).
+
+If `expand: true` and steps ≤ 4 and there is no cancel/subst visual,
+superpaper still uses `align`. This package is for when the rewrite
+must be *seen*.
+
+## Workflow
+
+1. `./scripts/preflight.sh`
+2. One rewrite path. Drop equivalent expansions.
+3. Roles: `keep` / `rewrite` / `cancel` / `intro` (already declared).
+4. Smallest grammar: `\sdstep` + `\sdreason`; add `\sdcancel` / `\sdsubst` only when they carry information.
+5. `./scripts/build.sh fig.tex` then audit `references/checklist.md`.
+
+## Output
+
+Editable TikZ. Build emits PDF, SVG, PNG, transparent PNG, thumbnail.
+
+Chinese: `\usepackage[cjk]{superderive}`. English: `[en]`.