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>

lib/openttd/protocol.py

@@ -5,6 +5,66 @@ from openttd_protocol.wire.tcp import TCPProtocol
 from openttd_protocol.wire.read import read_uint8, read_string, read_uint16, read_uint32
 from openttd_protocol.wire.exceptions import SocketClosed
 
+def write_varuint(buffer, value):
+    """Encode a non-negative integer using OpenTTD's UTF-8-like varuint scheme."""
+    if value < 0:
+        raise ValueError("write_varuint requires a non-negative value")
+    thresholds = [1 << 7, 1 << 14, 1 << 21, 1 << 28, 1 << 35, 1 << 42, 1 << 49, 1 << 56]
+    for extra, limit in enumerate(thresholds):
+        if value < limit:
+            header_ones = (0xFF << (8 - extra)) & 0xFF
+            header = header_ones | (value >> (extra * 8))
+            buffer.append(header)
+            for i in range(extra - 1, -1, -1):
+                buffer.append((value >> (i * 8)) & 0xFF)
+            return
+    buffer.append(0xFF)
+    for i in range(7, -1, -1):
+        buffer.append((value >> (i * 8)) & 0xFF)
+
+def read_varuint(data):
+    """Decode a varuint written by write_varuint. Returns (value, rest)."""
+    header = data[0]
+    mask = 0x80
+    extra = 0
+    while header & mask:
+        extra += 1
+        mask >>= 1
+    value = header & (0x7F >> extra)
+    rest = data[1:]
+    for i in range(extra):
+        value = (value << 8) | rest[i]
+    return value, rest[extra:]
+
+def write_varuint_signed(buffer, value):
+    """Encode a signed integer using zigzag + write_varuint."""
+    zigzag = (value << 1) ^ (-1 if value < 0 else 0)
+    write_varuint(buffer, zigzag)
+
+def read_varuint_signed(data):
+    """Decode a signed varuint written by write_varuint_signed. Returns (value, rest)."""
+    zigzag, rest = read_varuint(data)
+    value = (zigzag >> 1) ^ -(zigzag & 1)
+    return value, rest
+
+class GameCommand(IntEnum):
+    ChangeTimetable = 174
+    SetVehicleOnTime = 176
+    AutofillTimetable = 177
+    SetTimetableStart = 180
+
+class ModifyTimetableFlags(IntEnum):
+    WaitTime = 0
+    TravelTime = 1
+    TravelSpeed = 2
+    SetWaitFixed = 3
+    SetTravelFixed = 4
+    SetLeaveType = 5
+    AssignSchedule = 6
+
+class ModifyTimetableCtrlFlag(IntEnum):
+    ClearField = 1 << 0
+
 class PacketGameType(IntEnum):
     ServerFull = 0
     ServerBanned = 1
@@ -229,7 +289,25 @@ class OpenTTDProtocol(TCPProtocol):
     @staticmethod
     def receive_ServerExternalChat(source, data): return {}
     @staticmethod
-    def receive_ServerCommand(source, data): return {}
+    def receive_ServerCommand(source, data):
+        company, data = read_uint8(data)
+        cmd, data = read_uint16(data)
+        error_msg, data = read_uint16(data)
+        tile, data = read_uint32(data)
+        payload_len, data = read_uint16(data)
+        payload = data[:payload_len]
+        data = data[payload_len:]
+        callback, data = read_uint8(data)
+        callback_param = 0
+        if callback != 0:
+            callback_param, data = read_uint32(data)
+        frame, data = read_uint32(data)
+        my_cmd, _ = read_uint8(data)
+        return {
+            "company": company, "cmd": cmd, "error_msg": error_msg, "tile": tile,
+            "payload": payload, "callback": callback, "callback_param": callback_param,
+            "frame": frame, "my_cmd": bool(my_cmd)
+        }
     @staticmethod
     def receive_ServerFull(source, data): return {}
     @staticmethod