Keep host alive across audio device changes

[rustydomino]

Keep SoundBridgeHost running when no physical output devices are available,
and allow it to transition between idle and active operation as devices are
connected and disconnected.

Changes include:

• add explicit host lifecycle states
• remain running in an idle state when no output devices are available
• initialize the AudioEngine when devices appear
• stop the AudioEngine and heartbeat when the last device disappears
• serialize Core Audio device callbacks onto the main queue
• register device-change listeners from the common startup path
• make sleep/wake handling work in both active and idle states

Tested:

• normal startup: starting -> active
• no-device startup: starting -> idle
• device connection: idle -> active
• device removal: active -> idle
• reconnect: idle -> active
• active sleep/wake: AudioEngine stops and restarts successfully
• idle sleep/wake: remains idle without starting the AudioEngine

The existing signal-handler lifetime issue is separate from this change.