Class BoundaryApi

Install an already-constructed Boundary on two hosts: create the two BoundaryAnchors, wire boundary.anchorA/anchorB, and call each host's addFixture(anchor). Returns the boundary for fluent chaining.

Rejects when the boundary already has anchors wired (use destruct first to migrate). Rejects hostA === hostB (a Boundary connecting a room to itself has no use case in v1 and would confuse the getOtherHost walk).

Convenience: clone-or-construct a Boundary via StuffApi.create (the canonical async creation path) and install it on two hosts in one call. Mirrors the "factory + two endpoints" shape of Exitable.addBidirectionalExit.

Tear down a Boundary and its anchors. Routes through StuffApi.destruct so the proxy / shadow lifecycle runs; the Boundary's own onDestruct detaches the anchors from each host's getFixtures() and destructs them.