define a new ZoeSeatKit that doesn't create cycles

[Chris-Hibbert]

What is the Problem Being Solved?

When we create ZoeSeats, we accidentally create a cycle that prevents garbage collection.

Description of the Design

Leave the existing definition of ZoeSeatKits, so existing seats continue to function. Stop invoking its maker.

Create a new pair of exo classes that separate the userSeat from the rest of the kit, and which can break the cycle when the seat is exited. (See Avoiding Cycles In The First Place in #8401). Use the maker when making new seats.

Security Considerations

None. But don't lose existing seats that might be providing access to assets.

Scaling Considerations

Stops creating cyclic data structures that GC can't free.

Test Plan

verify that we don't create uncollectible cycles.

Upgrade Considerations

Requires a Zoe upgrade.