multiplayer stage 3: crew model, claim handshake and AI handover

A vehicle is not owned by one player: two people share a cab, so the session
needs a crew relation rather than an owner field. The server keeps it and is
the only one allowed to change it.

- network/session: vehicle_crew { entity_id, crew, ai_before_claim } and a
  crew_registry holding the roster. the host goes through exactly the same
  path as a remote peer (claim -> crew -> grant), only without transport
- CLAIM_VEHICLE / CLAIM_GRANTED / CLAIM_DENIED / LEAVE_VEHICLE / CREW_UPDATE;
  the server validates the vehicle exists, is present in the scenario and has
  room in the crew, and answers a refusal with a reason the lobby shows
- the cab of a claimed vehicle is created by the authority posting the
  ordinary replicated entervehicle, so every peer builds it and the vehicle is
  never moved. the claiming peer only walks in once the cab appears locally
- AI transitions follow the crew size: 0 -> 1 remembers whether an AI was
  really driving and takes the vehicle over, 1 -> 2 and 2 -> 1 change nothing,
  1 -> 0 hands the vehicle back but only if there was an AI driver to begin
  with. driver mode no longer fires aidriverdisable of its own in multiplayer
- losing a connection removes that peer from every crew it was on; a vehicle
  with somebody still on board carries on untouched
- the lobby lists who is on board, offers "join crew" for a vehicle another
  player already occupies, and lets a player step off

The AI and cab handling runs as a reconciliation pass rather than as edge
triggered actions, so a request that has not taken effect yet (the cab needs a
frame to appear) is simply repeated, with a cooldown to keep it off the wire.

network/message.h

@@ -16,6 +16,11 @@ struct message
 		REQUEST_COMMAND,
 		SERVER_REJECT,
 		VEHICLE_LIST,
+		CLAIM_VEHICLE,
+		CLAIM_GRANTED,
+		CLAIM_DENIED,
+		LEAVE_VEHICLE,
+		CREW_UPDATE,
 		TYPE_MAX
 	};
 
@@ -80,6 +85,63 @@ struct vehicle_list : public message
 	virtual void deserialize(std::istream &stream) override;
 };
 
+// a peer asking the server to be put on the crew of a vehicle
+struct claim_vehicle : public message
+{
+	claim_vehicle() : message(CLAIM_VEHICLE) {}
+
+	uint32_t entity_id{ ENTITY_NONE };
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
+// answer to a claim the server accepted
+struct claim_granted : public message
+{
+	claim_granted() : message(CLAIM_GRANTED) {}
+
+	uint32_t entity_id{ ENTITY_NONE };
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
+// answer to a claim the server refused, with a reason the lobby can show
+struct claim_denied : public message
+{
+	claim_denied() : message(CLAIM_DENIED) {}
+
+	uint32_t entity_id{ ENTITY_NONE };
+	std::string reason;
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
+// a peer stepping off the crew of a vehicle
+struct leave_vehicle : public message
+{
+	leave_vehicle() : message(LEAVE_VEHICLE) {}
+
+	uint32_t entity_id{ ENTITY_NONE };
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
+// authoritative crew of one vehicle, broadcast whenever it changes
+struct crew_update : public message
+{
+	crew_update() : message(CREW_UPDATE) {}
+
+	uint32_t entity_id{ ENTITY_NONE };
+	std::vector<PeerId> crew;
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
+};
+
 struct request_command : public message
 {
 	request_command(type_e type) : message(type) {}