test(11.1-06): branch-level tests for every docsite stage (94.8% coverage)

- load, render, emit, snippet, highlight, serve and checker branch tests;
  docs:build --check, docs:sync and docs:serve output tests in cmd/summer
- fix: a CRLF or BOM page is reported as such, not as a page without a
  frontmatter block (TestFrontmatterEncodingProblems)
- fix: src=#Type.Method finds a method with a parenthesised receiver
  (TestSnippetGenericsAndGroups, Box.Paren)
- fix: docs:serve adds its watches before announcing the server, so an
  edit made right after the serving line rebuilds (TestServeWatchRebuilds)

internal/docsite/load.go

@@ -369,7 +369,7 @@ func (s *site) loadGuides() ([]*Page, []Problem, error) {
 		}
 		fmRaw, body, bodyLine, ok := splitFrontmatter(raw)
 		if !ok {
-			fail(`the file must start with a "---" frontmatter block closed by a "---" line`)
+			fail(frontmatterBlockProblem(raw))
 			continue
 		}
 		fm, details := parseFrontmatter(fmRaw)
@@ -488,6 +488,20 @@ func splitFrontmatter(raw []byte) (fm, body []byte, bodyLine int, ok bool) {
 	return nil, nil, 0, false
 }
 
+// frontmatterBlockProblem explains why a page has no frontmatter block. A
+// byte order mark or CRLF line endings hide a block the file does have, so
+// they are named instead of the missing block.
+func frontmatterBlockProblem(raw []byte) string {
+	first, _, _ := bytes.Cut(raw, []byte("\n"))
+	switch {
+	case bytes.HasPrefix(raw, []byte("\xef\xbb\xbf")):
+		return "the file starts with a UTF-8 byte order mark; save it without one"
+	case bytes.HasSuffix(first, []byte("\r")):
+		return "the file uses CRLF line endings; save it with LF line endings"
+	}
+	return `the file must start with a "---" frontmatter block closed by a "---" line`
+}
+
 // parseFrontmatter decodes a frontmatter block strictly. It returns the
 // UI-SPEC problem details: unknown and missing fields first, then decode
 // errors.