The level meter only measured a stream a call had already opened, so settings
offered no way to check a mic before joining — the panel said as much.
Add startMicTest/stopMicTest on AudioManager: the processed input bus is
routed to the master output through a dedicated gain node, so the loopback can
be disconnected precisely. Settings had deliberately never opened the mic
itself; a mic test cannot honour that, so the test hands the mic back when it
stops.
Releasing needs two independent guards, because the user may join a call
mid-test: AudioManager only stops the exact stream it opened (identity check,
not a flag), and the caller must consent — the UI reads the call state, which
AudioManager cannot, as it does not import stores. Unmounting mid-test tears
the loopback down too.
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