`wait(stats=True)` behaves weird if written in the root of the `__main__` module

I was playing with BocPy and got some weird behaviour when using wait(stats=True) in the root of the __main__ module. Calling wait(stats=True) works just fine, but storing the result in a global variable stats = wait(stats=True) causes an exception AttributeError: 'NoneType' object has no attribute 'join'

Here is a the small example

from bocpy import Cown, when, wait

c = Cown([1, 2, 3])

def main():
    @when(c)
    def x(c):
        print(f"1: {hex(id(c.value))}: {c.value}")

if __name__ == "__main__":
    main()
wait(stats=True) # Is fine

if __name__ == "__main__":
    main()
stats = wait(stats=True) # Runtime Error

For me, this results in the following exception:

Traceback (most recent call last):
  File "<workspace>/bocpy/pyrona.py", line 11, in <module>
    main()
    ~~~~^^
  File "<workspace>/bocpy/pyrona.py", line 6, in main
    @when(c)
     ~~~~^^^
  File "<workspace>/bocpy/src/bocpy/behaviors.py", line 1990, in when_factory
    start(module=get_caller_module())
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<workspace>/bocpy/src/bocpy/behaviors.py", line 1960, in start
    BEHAVIORS.start(module)
    ~~~~~~~~~~~~~~~^^^^^^^^
  File "<workspace>/bocpy/src/bocpy/behaviors.py", line 1034, in start
    exec(
    ~~~~^
        compile(export.code, main_export_file, "exec"),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        main_export.__dict__,
        ^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "<bocpy:main:__main__>", line 9, in <module>
    stats = wait(stats=True)
  File "<workspace>/bocpy/src/bocpy/behaviors.py", line 2134, in wait
    BEHAVIORS.stop(_remaining())
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "<workspace>/bocpy/src/bocpy/behaviors.py", line 1390, in stop
    self.noticeboard.join(_remaining())
    ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'join'

This can be fixed by moving the wait into the main() function, but it's still a bit weird.

Start with src/bocpy/behaviors.py at wait(), stop(), when_factory(), and start(), following the traceback from the provided root-level example. Reproduce both wait(stats=True) forms and inspect why the second path reaches a None noticeboard. Done means the example no longer raises AttributeError and the wait result can be assigned successfully, with a regression test.