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/utils.py

@@ -0,0 +1,10 @@
+from __future__ import annotations
+
+
+def is_newer(a: int, b: int) -> bool:
+    """Return True if 16-bit sequence number a is newer than b (wrap-aware).
+
+    Uses half-range window on unsigned 16-bit arithmetic.
+    """
+    return ((a - b) & 0xFFFF) < 0x8000
+