Though the runtime hasn't been changed over in this patch (it was in the
local index at the time however), the test does now demonstrate that
using a `Started` the correctly typed `.pld` will codec correctly when
passed manually to `MsgCodec.encode/decode()`.
Despite not having the runtime ported to the new shuttle msg set
(meaning the mentioned test will fail without the runtime port patch),
I was able to get this first original test working that limits payload
packets as a `Msg.pld: NamespacePath`this as long as we spec
`enc/dec_hook()`s then the `Msg.pld` will be processed correctly as per:
https://jcristharif.com/msgspec/extending.html#mapping-to-from-native-types
in both the `Any` and `NamespacePath|None` spec cases.
^- turns out in this case -^ that the codec hooks only get invoked on
the unknown-fields NOT the entire `Struct`-msg.
A further gotcha was merging a `|None` into the `pld_spec` since this
test spawns a subactor and opens a context via `send_back_nsp()` and
that func has no explicit `return` - so of course it delivers
a `Return(pld=None)` which will fail if we only spec `NamespacePath`.
3a105e2830