Sweep the `examples/debugging/` set for basic typing: add `-> None`
to all 16 bare `async def main()`s and annotate the clean
single-line `open_nursery()` bindings as `tractor.ActorNursery`.
Kept to the unambiguous, runtime-safe cases (these breakpoint/crash
demos can't be run headless); the heterogeneous
multi-line/paren-group nursery bindings + `current_actor()` returns
are left for a later pass. Continues the examples-typing bullet in
#472.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Such that we audit the `shield=root_tn.cancel_scope.cancel_called,`
passed to `await debug._maybe_enter_pm()` in the `open_root_actor()`
exit handler block.
Gud Boi
Tyler Goodlet