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/transport.py

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+import asyncio
+from dataclasses import dataclass
+from typing import Awaitable, Callable, Optional, Tuple
+
+
+OnDatagram = Callable[[bytes, object], Awaitable[None]]
+
+
+@dataclass
+class TransportPeer:
+    addr: object  # opaque peer handle (e.g., QUIC session)
+
+
+class DatagramServerTransport:
+    async def send(self, data: bytes, peer: TransportPeer) -> None:
+        raise NotImplementedError
+
+    async def run(self) -> None:
+        raise NotImplementedError
+
+
+class InMemoryTransport(DatagramServerTransport):
+    """A test transport that loops datagrams back to registered peers."""
+
+    def __init__(self, on_datagram: OnDatagram):
+        self._on_datagram = on_datagram
+        self._peers: list[TransportPeer] = []
+
+    def register_peer(self, peer: TransportPeer) -> None:
+        self._peers.append(peer)
+
+    async def send(self, data: bytes, peer: TransportPeer) -> None:
+        # In-memory: deliver to all except sender to simulate broadcast domain
+        for p in self._peers:
+            if p is peer:
+                continue
+            await self._on_datagram(data, p)
+
+    async def run(self) -> None:
+        # Nothing to do in in-memory test transport
+        await asyncio.Future()
+
+
+class QuicWebTransportServer(DatagramServerTransport):
+    """Placeholder for a real WebTransport (HTTP/3) datagram server.
+
+    Integrate with aioquic or another QUIC library and invoke the provided
+    on_datagram callback when a datagram arrives.
+    """
+
+    def __init__(self, on_datagram: OnDatagram):
+        self._on_datagram = on_datagram
+
+    async def send(self, data: bytes, peer: TransportPeer) -> None:
+        raise NotImplementedError("QUIC server not implemented in skeleton")
+
+    async def run(self) -> None:
+        raise NotImplementedError("QUIC server not implemented in skeleton")
+