fix(zoe): add Far to periodObserver

[warner]

refs #2018

[warner]

Until we get marshal's "reject unmarked pass-by-reference objects" behavior landed (which can't happen until we've gone through all our code and marked everything with Far), we'll be playing a catch-up game. I'm watching the tree for new harden({ methods.. }) changes and will submit PRs for Far-ing them.

Once we get that behavior landed, tests will fail if new code fails to use Far, which will make it a lot more obvious :).