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/config.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass
+class ServerConfig:
+    width: int = 60
+    height: int = 40
+    tick_rate: int = 10  # TPS, server-controlled, live-configurable
+    wrap_edges: bool = False  # default off
+    apples_per_snake: int = 1  # min 1, max 12
+    apples_cap: int = 255  # absolute cap
+    compression_mode: str = "none"  # "none" | "deflate" (handshake-only)
+    players_max: int = 32
+
+    def validate_runtime(self) -> None:
+        if not (3 <= self.width <= 255 and 3 <= self.height <= 255):
+            raise ValueError("field size must be within 3..255")
+        if not (5 <= self.tick_rate <= 30):
+            raise ValueError("tick_rate must be 5..30 TPS")
+        if not (1 <= self.apples_per_snake <= 12):
+            raise ValueError("apples_per_snake must be 1..12")
+        if not (0 <= self.apples_cap <= 255):
+            raise ValueError("apples_cap must be 0..255")
+        if self.compression_mode not in ("none", "deflate"):
+            raise ValueError("compression_mode must be 'none' or 'deflate'")
+