pyspy: sampler thread shutdown blocks until next sample

looks like this code
```
let sampler_output = sampler.take_while(|_x| running.load(Ordering::Relaxed));
```
blocks until the next sample (up to 10ms with default config)
We should consider exposing the channel so that we don't wait for 10ms

Start at the sampler.take_while closure shown in the issue and read the sampler shutdown path, focusing on how the running flag interacts with waiting for the next sample. Confirm the default 10ms wait, then verify that stopping the sampler completes without unnecessarily waiting for another sample.

Written by the indexing model from the issue text.