[larsoner]

Fix a few minor bugs I came across:

1. Protect memset call for input-only streams
2. Add a __del__ for mixers that ensure they are closed. Otherwise a garbage-collected mixer could keep calling a callback with a freed state.
3. Have the cancel action keep the action alive in the meantime, otherwise the callback could potentially read freed memory
4. Probably the most useful for me personally -- raise a RuntimeError if you try to enqueue an action and it would otherwise be silently ignored because the queue is full.

Bumped the PortAudio dep while I was in there. I found these because I asked Claude Opus 5.5 to investigate queue / PortAudio use since I've occasionally in CIs seen failures related to a queue being full. Opus made some suggestions, I had it implement them, and I reviewed and vouch for them. They all seem like reasonable / useful hardenings to me.

(This repo has no agentic guidance and we probably don't need it yet, but happy to talk more about it if desired. In MNE-Python we've had some nice improvements in terms of efficiency and bugs by having experts use it IMO, so I'll use it a bit here, too, to make little fixes/improvements unless there are objections! I could for example have it add some minimal unit tests, which could help us find some bugs as well.)