multiplayer: a player works a train, not a car - and put the corrections right

Second live test. Four things, three of them the same mistake seen from
different angles: the session was reasoning about single vehicles where it
should have been reasoning about trains.

- a peer may now operate anything coupled into the set it is sitting in. This
  is what "cabchangebackward rejected: not a member of the crew of that
  vehicle" was: the crew was recorded against one car of a four-car unit, and
  the cab switch addressed another
- every vehicle gets a network id, not only the ones worth driving. Commands
  are addressed by that id, and a player has to be able to walk into any car
  of their train. "peer 3 cannot take en71-017rb_4: no such vehicle in this
  session" was the driving trailer of an EN71 - a full desk, no engine, and
  nobody assigned to it, so the old test missed it. Having a driving position
  is what counts now, engine or not
- the roster groups vehicles into the trains they are coupled into and the
  lobby lists one row per train. Coupling is worked out afresh every time, so
  a train that comes apart becomes two, and because a crew entry sits on the
  car its player occupies, each half keeps the people who were riding in it
- walking to another car of your own train is not a new claim and asks nothing
  of the crew capacity, which counts people per train

Positional correction never actually did anything: it identified a track by
name, and most track in a scenery has none, so the lookup returned nothing and
place_on_track() was never reached. The client sat 178 m out of place for
minutes, reporting four vehicles "put back on the rails" every time while
moving none of them. Tracks now travel as their index in the path table, which
comes from the scenery file and is therefore the same on every peer.

Also from the test:

- the camera jumped in and out of the cab on the client whenever the host
  reversed, because the correction was forcing the server's CabActive and
  CabOccupied onto it. Which cab somebody sits in is theirs alone - two people
  sharing a vehicle each have their own - so it is no longer replicated
- the correction no longer touches the control positions of the train the local
  player is working. Their crew's commands reach every peer on their own, and
  snapping a lever back a fraction of a second after it was moved is what made
  the reverser look dead
- the frame buffer was sized for the old lockstep, up to sixty frames deep.
  Every frame in it is input lag, so it is down to twelve with a target of one
- asking the AI of a vehicle to step aside is given five tries and then said
  once, instead of every thirty frames forever; and a resync request now has a
  real cooldown

network/session.h

@@ -34,6 +34,9 @@ struct vehicle_crew
 	// keeps reconcile() from re-posting the same request every single frame while the
 	// previous one is still on its way through the command queue
 	int action_cooldown{0};
+	// how many times we have asked the AI to hand this vehicle over. if it will not, that
+	// is worth saying once rather than asking forever
+	int ai_attempts{0};
 };
 
 enum class claim_result
@@ -74,6 +77,9 @@ public:
 
 	// client side: takes over the crew of one vehicle as published by the server
 	void mirror(NetworkEntityId Id, std::vector<PeerId> const &Crew);
+	// puts a peer on a vehicle without asking about capacity; used when somebody walks to
+	// another car of the train they are already working
+	void take_seat(PeerId Peer, NetworkEntityId Id);
 
 	// server side: brings the world in line with the crew roster - creates the cab of a
 	// claimed vehicle, hands the vehicle over from the AI and hands it back when the last
@@ -138,6 +144,10 @@ std::string describe(command_verdict Verdict);
 // simply holds every right - there is no separate path for the local participant
 command_verdict validate_command(PeerId Peer, user_command Command, uint32_t Recipient);
 
+// a player works a train, not a single car: they may operate anything coupled into the
+// set they are sitting in, which is what makes the cab switches of a multiple unit work
+bool may_control(PeerId Peer, NetworkEntityId Entity);
+
 // walking into a cab with the in-game key is the same request as pressing the button in
 // the lobby, so it goes through the crew registry rather than being treated as a command
 // that reaches the whole world. returns the vehicle it resolved to, if any