Convention sweep (user req): all `tractor.open_nursery()`
bindings in test + example code use `an: ActorNursery` (`n`,
`nursery` + several tractor-nurseries confusingly named `tn`
are renamed); `trio.open_nursery()` bindings stay `tn` (incl.
`concurrent_actors_primes.py`'s inner trio nursery, renamed
`n` -> `tn` to match).
Purely mechanical, function-scoped renames — prose "nursery"/
"an" in docstrings/comments untouched; func-arg kwargs like
`portal.run(func, n=value)` untouched.
Gate: renamed test modules green on `trio`; full debugger suite
(28p/6s) + example-runner (21p) green.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi
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