fix(11.1-07): find src= fences in the goldmark AST and refuse nested ones

- collectFences walks every fenced code block the renderer parses
- a nested src= fence is refused and is not extracted, synced or captioned
- checkSnippets runs on the same parse as the other checkers
- Sync rewrites drifted top-level fences and keeps the frontmatter

Deviation: TestSyncRewritesDrift's drifted src= fence sat inside a list item, which is now refused. The drift case is a two-space top-level fence so Sync still rewrites it (problem line 11).

internal/docsite/load.go

@@ -179,11 +179,6 @@ func assemble(opts Options) (*site, []Problem, error) {
 	if err != nil || s == nil {
 		return nil, problems, err
 	}
-	sp, err := s.checkSnippets()
-	if err != nil {
-		return nil, nil, err
-	}
-	problems = append(problems, sp...)
 	cp, err := s.checkContent()
 	if err != nil {
 		return nil, nil, err