Remove player from zone contents on disconnect

Player objects were removed from the players dict on quit/disconnect
but never removed from zone._contents, leaving ghost * markers on
other players' maps.

tests/test_commands.py

@@ -536,3 +536,41 @@ async def test_look_no_here_line_when_alone(player, test_zone):
 
     output = "".join(c[0][0] for c in player.writer.write.call_args_list)
     assert "Here:" not in output
+
+
+@pytest.mark.asyncio
+async def test_quit_removes_player_from_zone(monkeypatch):
+    """Quitting removes player from zone contents."""
+    from mudlib.commands import quit as quit_mod
+    from mudlib.player import players
+
+    monkeypatch.setattr(quit_mod, "save_player", lambda p: None)
+
+    terrain = [["." for _ in range(10)] for _ in range(10)]
+    zone = Zone(
+        name="qz",
+        width=10,
+        height=10,
+        toroidal=True,
+        terrain=terrain,
+        impassable=set(),
+    )
+    writer = MagicMock()
+    writer.write = MagicMock()
+    writer.drain = AsyncMock()
+    writer.close = MagicMock()
+    p = Player(
+        name="quitter",
+        location=zone,
+        x=5,
+        y=5,
+        reader=MagicMock(),
+        writer=writer,
+    )
+    players.clear()
+    players["quitter"] = p
+
+    assert p in zone._contents
+    await quit_mod.cmd_quit(p, "")
+    assert p not in zone._contents
+    assert "quitter" not in players