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

scripts/build.sh

@@ -35,9 +35,13 @@ if grep -qE "^(Overfull|Underfull) \\\\[hv]box" "$LOG"; then
   status=1
 fi
 if grep -q "Package superderive Warning" "$LOG"; then
-  echo "!! superderive warnings:" >&2
-  grep -m5 -A2 "Package superderive Warning" "$LOG" >&2
-  status=1
+  if grep -q "^% superderive-build: allow" "$SRC"; then
+    echo "   superderive warnings present (suppressed by superderive-build: allow)" >&2
+  else
+    echo "!! superderive warnings:" >&2
+    grep -m5 -A2 "Package superderive Warning" "$LOG" >&2
+    status=1
+  fi
 fi
 
 if command -v pdftocairo >/dev/null 2>&1; then