Sure makes console grokability a lot better by showing only the
customizeable fields.
Further, clean up `mk_codec()` a bunch by removing the `ipc_msg_spec`
param since we don't plan to support another msg-set (for now) which
allows cleaning out a buncha logic that was mostly just a source of
bugs..
Also,
- add temporary `log.info()` around codec application.
- throw in some sanity `assert`s to `limit_msg_spec()`.
- add but mask out the `extend_msg_spec()` idea since it seems `msgspec`
won't allow `Decoder.type` extensions when using a custom `dec_hook()`
for some extension type.. (not sure what approach to take here yet).
322e015d32