Introduce MainloopWaker for thread-safe pa_mainloop_wakeup

[orestisfl]

the standard Mainloop is !Send (it's wrapped in Rc), so today the only way for downstream code to interrupt a blocking Mainloop::iterate(true) from another thread is to reach into _inner, grab the raw *mut pa_mainloop, and invoke pa_mainloop_wakeup via hand-rolled unsafe extern "C". that single C function is documented as thread-safe (it just writes a byte to the mainloop's internal self-pipe), so a small safe abstraction is worthwhile.

• MainloopWaker: a Send + Sync + Clone handle wrapping the call.
• Mainloop::waker(): hands out a waker tied to this mainloop.

the handle stores the mainloop pointer behind a
Mutex<*mut MainloopInternal>. MainloopWaker::wakeup holds the mutex across pa_mainloop_wakeup; Mainloop::drop nulls the pointer under the same mutex before the underlying pa_mainloop is freed. wakers therefore never observe a freed pointer and become no-ops once the mainloop is gone.

Rc<MainloopInner> is left unchanged: the rest of the standard mainloop API is !Send by design, so the Arc-backed sidecar is scoped to the one operation that genuinely is cross-thread.

Came up in JakeStanger/ironbar#875