Add vehicle timetable get/set support

Timetables have no GameScript API surface, so this implements real
DoCommands over the game port (ClientCommand/ServerCommand) instead of
the Admin GameScript relay used for list_vehicles(): change_timetable(),
autofill_timetable(), set_timetable_start(), and set_vehicle_on_time()
send commands, while get_vehicle_timetable() reconstructs state purely
by observing ServerCommand broadcasts, since no query command exists.

Includes the custom varuint wire codec these commands require, a full
usage guide (docs/TIMETABLES.md), and a worked demo in main.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_protocol.py

@@ -38,7 +38,6 @@ def test_protocol_static_parsers():
     assert res["token"] == 7
     
     assert OpenTTDProtocol.receive_ServerExternalChat(None, b"") == {}
-    assert OpenTTDProtocol.receive_ServerCommand(None, b"") == {}
     assert OpenTTDProtocol.receive_ServerFull(None, b"") == {}
     assert OpenTTDProtocol.receive_ServerBanned(None, b"") == {}
     assert OpenTTDProtocol.receive_ClientIdentify(None, b"") == {}