Such that any `Return` is always capture for each ctx instance and set
in `._deliver_msg()` normally; ensures we can at least introspect for it
when missing (like in a recently discovered stream teardown race bug).
Yes this augments the already existing `._result` which is dedicated for
the `._outcome_msg.pld` in the non-error case; we might want to see if
there's a nicer way to directly proxy ref to that without getting the
pre-pld-decoded `Raw` form with `msgspec`?
Also use the new `ctx._pld_rx.recv_msg()` and drop assigning
`pld_rx._ctx`.
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