multiplayer stage 8: state correction and surviving a dropped connection

Two things were still missing once a session ran for a while: a peer that had
drifted apart from the authority had no way back short of restarting, and a
connection that blinked cost the player their seat and handed their vehicle to
the AI while the other driver was still sitting in it.

- a client that disagrees with the authoritative digest for 120 steps in a row
  asks to be put back in line, and the server answers with a fresh snapshot.
  There is a cooldown between requests, so a correction that needs a moment to
  take hold does not turn into a stream of them
- the server hands out a session token with its hello and remembers it. Present
  it again on the next connection and you come back as the same peer, with the
  same id
- a lost connection no longer removes the peer from its crews: the seat is held
  for a minute. Come back in time and nothing changed; stay away and the seat
  is given up properly, handing the vehicle back to the AI only when that
  leaves the crew empty and the AI was driving it in the first place
- after applying a snapshot a peer that finds itself on a crew walks into that
  cab, which is what makes a reconnect land the player back where they were
- the vehicle roster is sent to a joining peer right before its snapshot, so
  the crew section resolves to names instead of waiting for the next broadcast

network/session.h

@@ -55,8 +55,17 @@ public:
 	claim_result claim(PeerId Peer, NetworkEntityId Id);
 	// returns true when the peer really was part of the crew
 	bool leave(PeerId Peer, NetworkEntityId Id);
-	// removes a peer from every crew it belonged to, e.g. after a disconnect
+	// removes a peer from every crew it belonged to
 	void drop_peer(PeerId Peer);
+	// the peer lost its connection. its seat is held for a while, so that a short dropout
+	// does not empty a cab and hand a vehicle back to the AI under the other driver's nose
+	void suspend_peer(PeerId Peer);
+	// the peer made it back in time and keeps everything it had
+	void resume_peer(PeerId Peer);
+	// actually removes the peers whose grace period ran out
+	void expire_absences();
+	// true while the peer is gone but still holding its seat
+	bool is_absent(PeerId Peer) const;
 
 	std::vector<PeerId> crew_of(NetworkEntityId Id) const;
 	bool is_member(PeerId Peer, NetworkEntityId Id) const;
@@ -84,10 +93,21 @@ private:
 
 	std::unordered_map<NetworkEntityId, vehicle_crew> m_vehicles;
 	std::vector<NetworkEntityId> m_pending;
+	// peers that dropped out, with the moment their seat stops being held
+	std::unordered_map<PeerId, double> m_absent;
 };
 
 extern crew_registry Crews;
 
+// how long a disconnected peer keeps its place in the crew
+constexpr double RECONNECT_GRACE_SECONDS = 60.0;
+
+// resolves the identity of a connecting peer. an unknown or empty token means somebody
+// new and gets a fresh peer id together with a token of their own; a token this server
+// handed out before brings back the peer id that went with it, and with it the seat in
+// whatever crew the peer was on
+PeerId resolve_peer_identity(uint64_t &Token);
+
 // what a peer is allowed to ask the simulation for
 enum class command_verdict
 {