The editor's 'Device audio' now sends a MIDI Active-Sensing heartbeat (0xFE, every 250ms)
to the device while on. The firmware reads usb_midi.ports[0]; while it hears the heartbeat
(<1s) it shows a green 'MIDI' badge top-right and silences the buzzer (the computer plays);
~1s after it stops, it reverts to the buzzer and hides the badge. Manual MUTE_BUZZER still
works. Verified headless: host detected -> MIDI shown + buzzer duty 0; timeout -> reverts.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
cc56741483