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

run.py

@@ -0,0 +1,10 @@
+from server.server import main
+
+if __name__ == "__main__":
+    import asyncio
+
+    try:
+        asyncio.run(main())
+    except KeyboardInterrupt:
+        pass
+