OTP application entry point.
Reads and validates the AP_* environment (see AudioProxy.Config) before
starting anything, so an invalid configuration aborts the boot rather than
surfacing as a request-time failure.
The log is set up in the same breath, and before the first line is emitted:
AP_LOG_LEVEL is applied to the primary logger, and
AudioProxy.LogHandler attaches to the telemetry the router and the render
path produce. Neither is a process, so neither is in the supervision tree —
a telemetry handler runs in whichever process emits the event.