Server scaffold: protocol + config + transport abstraction + tick loop skeleton

- Protocol: PacketType, TLV Body types, QUIC varint, header, input_broadcast
  and config_update builders, 2-bit body bitpacking helper
- Config/model: live-config ServerConfig, basic GameState/Snake/Session
- Transport: InMemoryTransport placeholder and QUIC server stub
- Server: asyncio tick loop, periodic config_update broadcast, immediate
  input_broadcast relay; main entry and run.py

server/model.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Deque, Dict, List, Optional, Tuple
+from collections import deque
+
+from .protocol import Direction
+
+
+Coord = Tuple[int, int]
+
+
+@dataclass
+class Snake:
+    snake_id: int
+    head: Coord
+    direction: Direction
+    body: Deque[Coord] = field(default_factory=deque)  # includes head at index 0
+
+    @property
+    def length(self) -> int:
+        return len(self.body)
+
+
+@dataclass
+class PlayerSession:
+    player_id: int
+    name: str
+    color_id: int
+    peer: object  # transport-specific handle
+    input_seq: int = 0
+
+
+@dataclass
+class GameState:
+    width: int
+    height: int
+    snakes: Dict[int, Snake] = field(default_factory=dict)
+    apples: List[Coord] = field(default_factory=list)
+    tick: int = 0
+