fix(alsa): check PCM RT-safety before promotion

[roderickvd]

This should fix #1193 from the following issue:

When the user's ALSA device routes to PulseAudio through the pcm_pulse plugin, and cpal promotes the ALSA thread to realtime, then the core is dumped with SIGXCPU. The sequence is this:

1. realtime-dbus promotes the ALSA audio thread to RT via rtkit
2. rtkit grants SCHED_FIFO priority to that thread while enforcing RLIMIT_RTTIME
3. libpulse is not RT-safe and either spins or does expensive syscalls from the RT thread, burning through the RLIMIT_RTTIME budget
4. Kernel fires SIGXCPU and dumps core

This PR implements PCM type whitelisting for types that are RT-safe.

[roderickvd]

Note that this disables realtime promotion for pipewire-alsa even though that is RT-safe, unlike pulseaudio-alsa. This errs on the safe side: both are PCM type IOPLUG and we can't distinguish between them further.

Use the dedicated pipewire host and cpal feature flag if you want a realtime thread on PipeWire.