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/snippet.go

@@ -312,6 +312,9 @@ func funcKey(d *ast.FuncDecl) string {
 		case *ast.IndexListExpr:
 			t = x.X
 			continue
+		case *ast.ParenExpr:
+			t = x.X
+			continue
 		case *ast.Ident:
 			return x.Name + "." + d.Name.Name
 		}