leaveDoesNotMintACancellationWhenTeardownIsSlow

No cancellation may be minted when the transport's teardown SUSPENDS — the #2286 mechanism, reachable only with the injector.

leaveDoesNotReportFailureAsCancellation proves the obligation on teardowns that return promptly, which is every teardown a 2-peer in-memory harness can produce. The trap it cannot reach is the one that actually happened: a seam.close that suspends, bounded by a withTimeout, throwing a TimeoutCancellationException to its caller while that caller's job stays perfectly alive. SeamRoom.leave calls seam.close(...) unguarded today, so this is a live guard against someone later bounding it — not a regression test for a fix.

TeardownFault.Slow suspends through delay, so the whole thing runs in virtual time and costs no wall clock. A harness whose fabric does real I/O gets a virtual delay here too, which is the same trade FaultProfile.DelayAll already makes: what is being modelled is a teardown with a suspension point in it, not a slow socket.