Riding: skip the extra rider pass when nothing rides the player

The second position_riders() call after the player moves rebuilt the mob
index every tick for every client; run it only when the riding graph has
something on this player. Also forget passengers whose vehicle is removed,
and add the new field to main's second (studio) Remote initializer.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/game/remote.rs

@@ -1390,6 +1390,7 @@ impl Game<'_> {
             use_target: None,
             menu_extra: None,
             entities: HashMap::new(),
+            passenger_vehicles: HashMap::new(),
             lerps: HashMap::new(),
             cart_lerps: HashMap::new(),
             stepped: HashMap::new(),
@@ -3352,6 +3353,7 @@ impl Game<'_> {
             r.glowing.remove(&id);
             r.entities.remove(&id);
             r.passenger_vehicles.remove(&(id as u32));
+            r.passenger_vehicles.retain(|_, v| *v != id as u32);
             r.lerps.remove(&id);
             r.cart_lerps.remove(&id);
             r.stepped.remove(&id);
@@ -6857,8 +6859,14 @@ impl Game<'_> {
         }
         // A stand riding the player follows this move (tickPassenger).
         self.seat_own_stands();
-        // Other players on this player (possibly through a plugin seat) follow this tick too.
-        self.position_riders();
+        // Other players on this player (possibly through a plugin seat) follow this tick too;
+        // the full pass only when something rides this player.
+        if self.remote.as_ref().is_some_and(|r| {
+            let me = r.entity_id as u32;
+            r.passenger_vehicles.values().any(|&v| v == me)
+        }) {
+            self.position_riders();
+        }
         // The server owns health and hunger.
         self.apply_remote_health();
         self.tick_avatar(before, false);