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/test.sh

@@ -22,6 +22,16 @@ for f in "$ROOT"/tests/lint-invalid/*.tex; do
     echo "  ok    $name (rejected by lint)"
   fi
 done
+for f in "$ROOT"/tests/invalid/*.tex; do
+  [[ -e "$f" ]] || continue
+  name="$(basename "$f")"
+  if "$ROOT/scripts/build.sh" "$f" >/dev/null 2>&1; then
+    echo "  FAIL  $name (invalid fixture built)"
+    fail=$((fail+1))
+  else
+    echo "  ok    $name (rejected by package/build)"
+  fi
+done
 if [[ $fail -gt 0 ]]; then
   echo "$fail failing check(s)" >&2
   exit 1