feat: add superderive stepwise derivation figures

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

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).