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

tests/invalid/empty-row.tex

@@ -0,0 +1,5 @@
+\usepackage[en]{superderive}
+\begin{document}\begin{tikzpicture}
+\sdrow{R1}
+\sdrowend
+\end{tikzpicture}\end{document}

tests/invalid/step-outside-row.tex

@@ -0,0 +1,4 @@
+\usepackage[en]{superderive}
+\begin{document}\begin{tikzpicture}
+\sdstep{s1}{a}{b}
+\end{tikzpicture}\end{document}

tests/invalid/two-contrast-pkg.tex

@@ -0,0 +1,7 @@
+\usepackage[en]{superderive}
+\begin{document}\begin{tikzpicture}
+\sdrow{R1}
+  \sdcol{c1}{a}{b}
+  \sdcol{c2}{c}{d}
+\sdrowend
+\end{tikzpicture}\end{document}

tests/invalid/undeclared-role.tex

@@ -0,0 +1,6 @@
+\usepackage[en]{superderive}
+\begin{document}\begin{tikzpicture}
+\sdrow{R1}
+  \sdbox[role=nosuch]{a}{x}
+\sdrowend
+\end{tikzpicture}\end{document}