Finishes the anyhow removal for the parts this branch adds: the status
bucket's open-or-create and the connected-client precondition. Two
variants, one of them behind the `kv` feature because the error type it
wraps does not exist without it — the error enum respects the same gate
the module does.
NotConnected is deliberately distinct from Connect: one is a connect that
was attempted and refused, the other is a request made before any
connection exists. The first is a deployment problem and the second is a
caller-ordering one, which is the whole reason a caller wants an enum
rather than a string.
The controller's `store` now returns the queue client's error rather than
an anyhow one: `OnceCell::get_or_try_init` takes its error type from the
closure, so widening there would mean converting inside the closure for
no gain. `view` `?`s it and anyhow converts at that boundary — the
library keeps a typed error, the binary keeps anyhow, and no call site
pays for the split.
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