Inside a new `.trionics._beg` and exposed from the subpkg ns in
anticipation of the `strict_exception_groups=False` being removed by
`trio` in py 3.15.
Notes,
- mk an embedded single-exc "extractor" using a `BaseExceptionGroup.exceptions` length
check, when 1 return the lone child.
- use the above in a new `@acm`, async bc it's most likely to be composed in an
`async with` tuple-style sequence block, called `collapse_eg()` which
acts a one line "absorber" for when the above mentioned flag is no
logner supported by `trio.open_nursery()`.
All untested atm fwiw.. but soon to be used in our test suite(s) likely!
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