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>
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