Zone.Entities read-only outside Zone: Admit and Remove are the only membership writers

Entities is a MemberView with no mutating member; Admit installs the death watch with the
membership and refuses a member already present, Remove takes it away with the membership.
The Reset and duplicate-Add bypass paths of the death watch cannot be spelled any more.
Tests move to Admit/Remove; Admit also activates, so the explicit Activate calls that followed
a raw Add are dropped. Unity call sites route through Zone.Remove.

Builds: unverified at this commit (verified in the job at the head).

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

Assets/Scripts/ServerShared/Entity.cs

@@ -1507,7 +1507,7 @@ public abstract class Entity
         {
             bay.DockedShip = ship;
             ship.SetParent(this);
-            Zone.Entities.Remove(ship);
+            Zone.Remove(ship);
             ship.Deactivate();
             ship.Docked.OnNext(this);
         }