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/snippet.go

@@ -452,6 +452,11 @@ type drift struct {
 // its container marker, so the fence is refused instead of extracted.
 const nestedSrcMessage = "src= code block must be a top-level block of the page, not inside a callout, blockquote or list item"
 
+// readmeSrcMessage is the problem for a src= fence in a module README.
+// Those fences are rendered as written and never compared, so a src=
+// there would claim a check that does not run.
+const readmeSrcMessage = "module README code blocks are rendered as written; src= is verified only in docs/ pages"
+
 // checkFences verifies every src= fence in fences. lines are the body
 // lines the fences index into. lineBase is the 1-based file line of
 // lines[0]; file is the display path for problems.
@@ -513,19 +518,30 @@ func unindent(l string, n int) string {
 	return l[i:]
 }
 
-// checkSnippets verifies the src= fences of every docs page. Module
-// READMEs are skipped here; a later check refuses src= on those pages
-// because their fences are rendered as written.
+// checkSnippets verifies the src= fences of every page. A docs/ page is
+// compared with its source. A module README is rendered as written, so
+// any src= fence there is refused instead of checked.
 func (s *site) checkSnippets(docs []parsedDoc) ([]Problem, error) {
 	var problems []Problem
 	for _, d := range docs {
-		if d.page == nil || d.page.Module != "" {
+		if d.page == nil {
 			continue
 		}
 		fences, err := collectFences(d.doc, d.body)
 		if err != nil {
 			return nil, err
 		}
+		if d.page.Module != "" {
+			for _, f := range fences {
+				ref, ok := ParseSrc(f.info)
+				if !ok {
+					continue
+				}
+				problems = append(problems, Problem{File: d.file, Line: d.line + f.line, Rule: "snippet",
+					Message: ref.String() + ": " + readmeSrcMessage})
+			}
+			continue
+		}
 		_, ps, err := checkFences(s.opts.Root, d.file, strings.Split(string(d.body), "\n"), d.line, fences)
 		if err != nil {
 			return nil, err