Toggling speaker output flipped the route back and forth indefinitely. Two
loops, both of which made a rebuild produce the condition for the next one.
Reconfiguring the session moves the route, and moving the route is reported
back through RouteChangeNotification. Forcing the speaker takes a headset out
of the route, which arrives as OldDeviceUnavailable, and releasing it brings
the headset back as NewDeviceAvailable; neither is among the reasons the
handler filters, so each rebuild answered its own echo with another rebuild.
Nothing compared the reported route against the route the live graph was
actually built on.
Record that route at the end of Apply, once the session is configured, and
rebuild only when a reported change differs from it; notifications that arrive
while Apply is still running describe the change Apply is itself making and are
ignored outright. The decision is AudioRouteWatcher in VoiceCat.Core, which is
platform-agnostic and tested, following ControlPathWatcher; the route identity
it compares is supplied by the caller, on iOS the UIDs of the current route's
ports. A graph whose route is unchanged but broken is still the stall
watchdog's to catch.
The port override was also re-asserted on every Apply, so where the system
wanted to hand output back to a connected headset each rebuild forced it to the
speaker again and the resulting route change drove the next rebuild. It is now
the one-shot request it should always have been, issued by the toggle alone;
the DefaultToSpeaker category option is the part that persists across rebuilds.
Also updates the route test from 724f7e9, which asserted the voice-chat preset
clearing the speaker flag and the absence of the port override. Both were
deliberately removed when speaker output became orthogonal to the preset, and
the test should have been updated with them.
