Server: implement JOIN/JOIN_ACK/JOIN_DENY handling, input parsing/relay, spawn logic, apples maintenance; fix InMemoryTransport to address specific peer; add state_full encoder

- Protocol: join parser, join_ack/deny builders, input parser, state_full builder
- Server: on_datagram dispatch, spawn per rules (prefer length 3 else 1), join deny if no cell, immediate input_broadcast relay
- Model: occupancy map and helpers
- Transport: deliver to specified peer in in-memory mode

server/model.py

@@ -38,4 +38,18 @@ class GameState:
     snakes: Dict[int, Snake] = field(default_factory=dict)
     apples: List[Coord] = field(default_factory=list)
     tick: int = 0
+    occupancy: Dict[Coord, Tuple[int, int]] = field(default_factory=dict)  # cell -> (snake_id, index)
 
+    def in_bounds(self, x: int, y: int) -> bool:
+        return 0 <= x < self.width and 0 <= y < self.height
+
+    def cell_free(self, x: int, y: int) -> bool:
+        return (x, y) not in self.occupancy
+
+    def occupy_snake(self, snake: Snake) -> None:
+        for idx, (cx, cy) in enumerate(snake.body):
+            self.occupancy[(cx, cy)] = (snake.snake_id, idx)
+
+    def clear_snake(self, snake: Snake) -> None:
+        for (cx, cy) in list(snake.body):
+            self.occupancy.pop((cx, cy), None)