multiplayer stage 2: peer identity, stable vehicle ids and a session lobby

TTrain::id() numbers cabs with a local counter, in the order each peer happens
to create them, so it cannot address a vehicle across the session. Neither was
there any way for a player to see what is available to drive.

- network/entities: PeerId (the host is peer 1, remote peers are numbered from
  2, so it is a participant rather than a special case) and NetworkEntityId,
  plus a registry that numbers the drivable vehicles of the loaded scenario in
  name order and resolves ids back to TDynamicObject
- the server owns the numbering and publishes it as VEHICLE_LIST - name, crew
  occupancy, crew capacity, AI state, availability - on every change and at
  least every 120 frames, out of band so it stays out of the replay backbuffer;
  a client binds the received ids to its own copy of the scenario by name
- SERVER_HELLO hands the joining peer its PeerId
- a multiplayer lobby listing that roster, with vehicles taken by other players
  shown as such rather than hidden; entering one goes through the existing
  entervehicle command, the vehicle is never moved

The lobby lives inside the driver mode instead of a mode of its own: the world
has to keep running while somebody is choosing (a client stops consuming the
server's frames the moment its mode is not a command processor), and the same
screen is what a player needs to change vehicles later on.

Crew occupancy is still derived from the presence of a cab instance; the real
crew model, claim handshake and AI takeover follow in stage 3.

network/message.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "network/message.h"
+#include "network/entities.h"
 #include "input/command.h"
 #include <queue>
 
@@ -14,6 +15,7 @@ struct message
 		FRAME_INFO,
 		REQUEST_COMMAND,
 		SERVER_REJECT,
+		VEHICLE_LIST,
 		TYPE_MAX
 	};
 
@@ -59,6 +61,20 @@ struct server_hello : public message
     std::string scenario;
 	// build identification of the server, informational only
 	std::string app_version;
+	// identity assigned to the joining peer for the rest of the session
+	uint32_t peer_id{ PEER_NONE };
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
+// authoritative roster of the vehicles a player may take over, broadcast by the server
+// whenever it changes (and periodically, so that a peer which just went active gets one)
+struct vehicle_list : public message
+{
+	vehicle_list() : message(VEHICLE_LIST) {}
+
+	std::vector<vehicle_entry> vehicles;
 
 	virtual void serialize(std::ostream &stream) const override;
 	virtual void deserialize(std::istream &stream) override;