feat: fill test coverage gaps + fix \sdcol i18n labels

- examples/subst-chain.tex: substitution walkthrough with \sdbox/\sdsubst/\sdstep
- examples/antipatterns.tex: four compile-clean-but-teach-false figures
- tests/invalid/step-outside-row.tex, empty-row.tex, undeclared-role.tex,
  two-contrast-pkg.tex: negative fixtures for package-level warnings
- test.sh: add tests/invalid/ loop
- build.sh: allow superderive-build: allow directive to suppress
  intentional warnings in antipatterns galleries
- superderive.sty: \sdcol respects [en] option for illegal/legal labels
- references/api.md: complete rewrite, all ~12 macros documented

examples/subst-chain.tex

@@ -0,0 +1,23 @@
+% Golden example -- a subst chain that must be seen, not just aligned.
+%   ../scripts/build.sh subst-chain.tex
+\documentclass[border=10pt]{standalone}
+\usepackage[cjk]{superderive}
+
+\sdsetrole{live}{sdTeal}
+
+\begin{document}
+\begin{tikzpicture}
+\sdstage{D1}{换元让积分内部对齐}
+\sdrow{R1}
+  \sdbox[role=keep]{x}{$u$}
+  \sdsubst[role=live]{s1}{u}{x^2}
+  \sdstep[role=rewrite]{s2}{\int_0^1 x\,e^{x^2}\,dx}{\tfrac12\int_0^1 e^u\,du}
+\sdrowend
+\sdbbox{all}
+\sdtopformula{F}{$\int_0^1 x\,e^{x^2}\,dx = \tfrac12\int_0^1 e^u\,du$}
+\sdmeaningbox{mb}{120mm}{all}
+  {换元是恒等改写，不改值}
+  {令 $u=x^2$，则 $du=2x\,dx$}
+  {上下限随 $u$ 一起换掉}
+\end{tikzpicture}
+\end{document}