Map empty Enter to ZSCII 13 in read_char

MudInputStream.read_char() returned 0 for empty input, which no game
recognizes as a valid keypress. Now returns 13 (Enter/Return) so
"press any key" prompts like Curses' intro work from a MUD client.

src/mudlib/zmachine/mud_ui.py

@@ -107,7 +107,9 @@ class MudInputStream(zstream.ZInputStream):
         text = self._input_queue.get()
         if text:
             return ord(text[0])
-        return 0
+        # Player hit Enter with no text — return 13 (ZSCII Enter/Return)
+        # so "press any key" prompts work in a line-oriented MUD client.
+        return 13
 
     def feed(self, text: str):
         self._input_queue.put(text)