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/transport.py

@@ -32,11 +32,9 @@ class InMemoryTransport(DatagramServerTransport):
         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)
+        # In-memory: deliver only to the addressed peer
+        if peer in self._peers:
+            await self._on_datagram(data, peer)
 
     async def run(self) -> None:
         # Nothing to do in in-memory test transport
@@ -58,4 +56,3 @@ class QuicWebTransportServer(DatagramServerTransport):
 
     async def run(self) -> None:
         raise NotImplementedError("QUIC server not implemented in skeleton")
-