So we can parametrize in various toggles to `main()` including,
- `child_errors_mid_stream: bool` which now also drives whether an
additional, and otherwise non-affecting, `_tn` is allocated in
the `finite_stream_to_rent()` subtask, only in the early stream
termination case does it seem to produce a masked outcome?
* see surrounding notes within.
- `raise_unmasked: bool` to toggle whether the embedded unmasker fn
will actually raise the masked user RTE; this enables demoing the
masked outcomes via simple switch and makes it easy to wrap them
as `pytest.xfail()` outcomes.
Also in support,
- use `.trionics.collapse_eg()` around the root tn to ensure when
unmasking we can catch the EG-unwrapped RTE easily from a test.
- flip stream `msg` logs to `.debug()` to reduce console noise.
- tweak mod's script iface to report/trace unexpected non-RTEs.
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