decide: ring-redirect a pool session whose host is between replicas #5267

User Story

I work on OpenShell's gateway HA session placement (#4223) and need this edge settled so shared-supervisor rollouts keep sessions on the replica serving their supervisor.

Description

Under supervisor_topology: gateway (#5052), a pool sandbox session dials the replica that accepted its host and follows the host when it reattaches elsewhere. When a host stream ends (for example an expiring supervisor token), its sessions prefer their home replica for GATEWAY_HOME_WAIT (5 s), then fall back to dialing the Service. If such a session lands on a replica that is not the ring owner for its sandbox, preferred_peer_redirect redirects it to the ring owner, which is not where its host is or will reattach. The session then sits on a different replica than its supervisor until the host moves again.

Gateway-side pool session placement was deliberately removed in #5052 ("pool sessions will dial their supervisor's gateway directly instead"), so this is a design question rather than a bug report: is the 5-second window plus host-follow enough, or should the gateway stop ring-redirecting sessions that belong to a shared supervisor?

Context

• Found while rescoping the placement metrics in feat(server): export supervisor session placement metrics #4692; the redirect path itself is unchanged there.
• RFC 0018 section 2 separates the routing owner from the enforcement owner; pool sessions follow their enforcement owner's host.

Definition of Done

• A decision on whether pool sessions should be exempt from ring redirects while hostless and, if so, where that knowledge lives (gateway, supervisor, or both)