Mobs: hand the ticked mob list back, not a copy of it

tick_creature_entities takes self.mobs out for the mob loop and appended it
back to the then empty list: Vec::append on an empty Vec copies every Mob
into a fresh buffer. With a server's 2000 mobs that is a multi-megabyte
allocation (mimalloc's huge-page path, madvise) and a memmove each tick,
1.5 % of the tick thread in a 60-player profile. Nothing adds to self.mobs
during the loop, so the list now moves back; the append stays for the case
where something did.

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

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

@@ -853,7 +853,13 @@ impl Game<'_> {
             bindings.retain(|id, _| live.contains(id));
             host.mob_players = bindings;
         }
-        self.mobs.append(&mut mobs);
+        // The ticked list goes back whole: appended to an empty list, a
+        // server's thousands of mobs were copied into a new buffer each tick.
+        if self.mobs.is_empty() {
+            self.mobs = mobs;
+        } else {
+            self.mobs.append(&mut mobs);
+        }
         self.died = died;
         // ... and entity event 60 goes out as it is: the poof.
         for (id, at, width, height) in gone {