Zone: wait for last tick's belt tasks before clearing the orbit cache

Update cleared `_updatedOrbits` and only then waited on the previous tick's
asteroid-belt tasks, which read the cache through GetOrbitPosition. A task
still running could see the set mid-clear ("concurrent update ... corrupted its
state") or recompute an orbit into last tick's position. The wait now comes
first.

Test (Soul's orbit counter, committed): 3000 back-to-back 0.02 s ticks of a
quiet arena, and no orbit stands still against its parent. Clearing first gave
500+ stalls or the corruption exception. Measured against the parent because a
moon can genuinely pause in zone space: one such reading (tick 1321, a moon
whose motion cancels its planet's) is real motion, not the race.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/Zone.cs

@@ -172,10 +172,11 @@ public class Zone
     public void Update(float deltaTime)
     {
         _time += deltaTime;
-        _updatedOrbits.Clear();
+        // Last tick's belt tasks read the orbit cache; they finish before it is cleared.
         foreach (var t in BeltUpdates)
             t.Wait();
         BeltUpdates.Clear();
+        _updatedOrbits.Clear();
         foreach (var orbit in Orbits)
         {
             orbit.Value.PreviousPosition = orbit.Value.Position;