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/antipatterns.tex

@@ -0,0 +1,46 @@
+% Anti-patterns -- figures that compile cleanly and still teach something false.
+%   ../scripts/build.sh antipatterns.tex
+% superderive-lint: allow-multiple-contrast
+% superderive-build: allow
+\documentclass[border=10pt]{standalone}
+\usepackage[cjk]{superderive}
+
+\begin{document}
+\begin{tikzpicture}
+\sdstage{AP}{四张反例}
+\sdrow{R1}
+  % AP1: \sdsubst 只填了 from，to 也是同一个 -- 改写没有产生任何变化
+  \sdsubst[role=rewrite]{ap1}{x+y}{x+y}
+  \sdreason{ap1}{subst 的 from 和 to 相同，改写为空}
+\sdrowend
+
+\sdrow{R2}
+  % AP2: \sdcancel 划线后旁边又写了一遍同样的东西 -- 读者不知道划掉的是哪一部分
+  \sdcancel[role=cancel]{ap2a}{x+y}
+  \sdbox[role=keep]{ap2b}{x+y}
+  \sdreason{ap2a}{cancel 后面紧跟同一式子，等于没有 cancel}
+\sdrowend
+
+\sdrow{R3}
+  % AP3: 同一行里塞了五组 role，hue budget 超了
+  \sdbox[role=keep]{ap3a}{a}
+  \sdbox[role=rewrite]{ap3b}{b}
+  \sdbox[role=cancel]{ap3c}{c}
+  \sdbox[role=intro]{ap3d}{d}
+  \sdbox[role=neutral]{ap3e}{e}
+\sdrowend
+
+\sdrow{R4}
+  % AP4: 两个 contrast pair，第二个是多余的
+  \sdcol{ap4a}{x+y}{x+y}
+  \sdcol{ap4b}{a+b}{a+b}
+\sdrowend
+
+\sdbbox{all}
+\sdtopformula{F}{反例见 \texttt{references/antipatterns.md}}
+\sdmeaningbox{mb}{140mm}{all}
+  {subst from=to；cancel 后重复；row 内五色；两个 sdcol}
+  {每一行都符合语法，但每一行都在说谎}
+  {参考 antipatterns.md 的对应规则}
+\end{tikzpicture}
+\end{document}

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}