lib/encoding/utf8+test: add strerror per Hare

lib/encoding/utf8/utf8.ww

@@ -29,6 +29,12 @@ export type more = void;
 // ref/hare/encoding/utf8/types.ha:9 — invalid UTF-8 sequence.
 export type invalid = !void;
 
+// ref/hare/encoding/utf8/types.ha:12 — fixed message; `invalid` carries
+// no payload, so the rendering is constant.
+export fn strerror(err: invalid) str = {
+	return "Invalid UTF-8";
+};
+
 // `done` is not a built-in singleton in ww (Hare ships it as part of
 // the type system). Plain `void` (not `!void`): end-of-input is a
 // continuation signal, not an error. lib/io spells its EOF the same