fix(lexer): tokenise the flag separator and handle NUL and invalid UTF-8

Assisted-by: GLM 5.3

token/token.go

@@ -42,6 +42,7 @@ const (
 	Arrow  // ->
 	At     // @
 	Hash   // #
+	Pipe   // |
 )
 
 var kindNames = map[Kind]string{
@@ -69,6 +70,7 @@ var kindNames = map[Kind]string{
 	Arrow:   "->",
 	At:      "@",
 	Hash:    "#",
+	Pipe:    "|",
 }
 
 // String returns a human-readable name for the kind.