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>