fix(coreaudio): aggregate-device uuid collision + auto start to true

[umerhd]

Two independent bugs in macOS LoopbackDevice, one commit each.

1. The aggregate-device UID was a hardcoded string literal, so concurrent LoopbackDevice instances collide on the same identity, and a crash before Drop leaves a stale registration that blocks the next launch. Suffix the UID with PID and a per-process atomic counter.
2. kAudioAggregateDeviceTapAutoStartKey was set to false, so the sub-tap is not engaged when the aggregate device opens. The input stream looks live (valid sample rate, valid timestamps) but every buffer is silence. Flip to true.