fix(11.1-07): check go fences, README src= and shell fences from the AST

- goLang follows the highlighter's chroma lookup, so golang and main.go need src=
- a src= fence in a module README is refused and never captioned
- shell fences inside callouts and lists are command-checked
- a fence with four or more leading spaces is an indented code block

internal/docsite/check_commands.go

@@ -86,18 +86,16 @@ func (s *site) checkCommands(docs []parsedDoc) ([]Problem, error) {
 		for _, span := range codeSpans(d.doc, d.body, d.line) {
 			check(d, span.line, span.text)
 		}
-		lines := strings.Split(string(d.body), "\n")
-		for _, f := range scanFences(lines) {
-			lang, _, _ := strings.Cut(f.info, " ")
-			if !slices.Contains(shellLangs, lang) {
+		fences, err := collectFences(d.doc, d.body)
+		if err != nil {
+			return nil, err
+		}
+		for _, f := range fences {
+			if !slices.Contains(shellLangs, f.lang) {
 				continue
 			}
-			end := f.close
-			if end < 0 {
-				end = len(lines)
-			}
-			for i := f.open + 1; i < end; i++ {
-				check(d, d.line+i, lines[i])
+			for _, line := range f.code {
+				check(d, d.line+line.line, line.text)
 			}
 		}
 	}