Suppress upper window writes in MudScreen to fix Lost Pig output

V5+ games write room names to the upper window (status line) via
select_window(1). Since select_window was a no-op, status line text
leaked into the main output buffer, causing ">Outside" on prompt lines.

Track the active window and only buffer writes to window 0 (lower).

src/mudlib/zmachine/mud_ui.py

@@ -21,6 +21,7 @@ class MudScreen(zscreen.ZScreen):
     def __init__(self):
         super().__init__()
         self._buffer = []
+        self._active_window = 0  # 0=lower (main text), 1=upper (status line)
         self._columns = 80
         self._rows = zscreen.INFINITE_ROWS
         self.features = {
@@ -31,6 +32,7 @@ class MudScreen(zscreen.ZScreen):
         }
 
     def write(self, string):
+        if self._active_window == 0:
             self._buffer.append(string)
 
     def flush(self) -> str:
@@ -39,10 +41,10 @@ class MudScreen(zscreen.ZScreen):
         return result
 
     def split_window(self, height):
-        logger.debug(f"split_window({height}) - no-op")
+        logger.debug(f"split_window({height})")
 
     def select_window(self, window):
-        logger.debug(f"select_window({window}) - no-op")
+        self._active_window = window
 
     def set_cursor_position(self, x, y):
         logger.debug(f"set_cursor_position({x}, {y}) - no-op")

tests/test_embedded_if.py

@@ -47,6 +47,37 @@ def test_mud_screen_flush_clears_buffer():
     assert second == ""
 
 
+def test_mud_screen_suppresses_upper_window_writes():
+    """MudScreen discards writes to upper window (status line)."""
+    screen = MudScreen()
+    screen.write("before ")
+    screen.select_window(1)  # upper window
+    screen.write("STATUS LINE")
+    screen.select_window(0)  # back to lower window
+    screen.write("after")
+    output = screen.flush()
+    assert output == "before after"
+
+
+def test_mud_screen_lower_window_is_default():
+    """MudScreen starts with lower window active, writes are captured."""
+    screen = MudScreen()
+    screen.write("hello")
+    assert screen.flush() == "hello"
+
+
+def test_mud_screen_upper_window_multiple_writes():
+    """MudScreen discards all writes while upper window is active."""
+    screen = MudScreen()
+    screen.select_window(1)
+    screen.write("Room Name")
+    screen.write(" | Score: 0")
+    screen.select_window(0)
+    screen.write("You are in a dark room.\n")
+    output = screen.flush()
+    assert output == "You are in a dark room.\n"
+
+
 def test_mud_input_stream_feed_and_read():
     """MudInputStream feed and read_line work with threading."""
     stream = MudInputStream()