feat: add superderive stepwise derivation figures

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

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`.