- set `._state._ctxvar_Context` just after `StartAck` inside
`open_context_from_portal()` so that `current_ipc_ctx()` always
works on the 'parent' side.
- always set `.canceller` to any `MsgTypeError.src_uid` and otherwise to
any maybe-detected `.src_uid` (i.e. for RAEs).
- always set `.canceller` to us when we rx a ctxc which reports us as
its canceller; this is a sanity check on definite "self cancellation".
- adjust `._is_self_cancelled()` logic to only be `True` when
`._remote_error` is both a ctxc with a `.canceller` set to us AND
when `Context.canceller` is also set to us (since the change above)
as a little bit of extra rigor.
- fill-in/fix some `.repr_state` edge cases:
- merge self-vs.-peer ctxc cases to one block and distinguish via
nested `._is_self_cancelled()` check.
- set 'errored' for all exception matched cases despite `.canceller`.
- add pre-`Return` phase statuses:
|_'pre-started' and 'syncing-to-child' depending on side and when
`._stream` has not (yet) been set.
|_'streaming' and 'streaming-finished' depending on side when
`._stream` is set and whether it was stopped/closed.
- tweak drainage log-message to use "outcome" instead of "result".
- use new `.devx.pformat.pformat_cs()` inside `_maybe_cancel_and_set_remote_error()`
but, IFF the log level is at least 'cancel'.
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