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/serve.go

@@ -49,6 +49,15 @@ func Serve(ctx context.Context, opts Options, addr string, allowRemote bool, out
 		return errors.New("docs:serve: build failed")
 	}
 
+	// The watches are in place before the serving line is printed, so an
+	// edit made as soon as the server is announced triggers a rebuild.
+	w, err := fsnotify.NewWatcher()
+	if err != nil {
+		return fmt.Errorf("docs:serve: watcher: %w", err)
+	}
+	defer w.Close()
+	s.addWatches(w)
+
 	ln, err := net.Listen("tcp", addr)
 	if err != nil {
 		return fmt.Errorf("docs:serve: %w", err)
@@ -61,7 +70,7 @@ func Serve(ctx context.Context, opts Options, addr string, allowRemote bool, out
 	watchErr := make(chan error, 1)
 	watchCtx, stopWatch := context.WithCancel(ctx)
 	defer stopWatch()
-	go func() { watchErr <- s.watch(watchCtx) }()
+	go func() { watchErr <- s.watch(watchCtx, w) }()
 
 	select {
 	case <-ctx.Done():
@@ -149,15 +158,8 @@ func (s *server) rebuild() bool {
 }
 
 // watch rebuilds on changes under Src, modules/ and every src= source
-// directory, debounced.
-func (s *server) watch(ctx context.Context) error {
-	w, err := fsnotify.NewWatcher()
-	if err != nil {
-		return fmt.Errorf("docs:serve: watcher: %w", err)
-	}
-	defer w.Close()
-	s.addWatches(w)
-
+// directory, debounced. The caller owns w and has added the first watches.
+func (s *server) watch(ctx context.Context, w *fsnotify.Watcher) error {
 	var timer *time.Timer
 	fire := make(chan struct{}, 1)
 	for {