v4.7-alpha.9

Fix dock audio glitch (click/pop every ~1s) by suppressing HID TX during source streaming.

Root cause: The dock polls for track info via USB HID every ~1 second. Each HID IN response caused the dock's USB host controller to drop the concurrent isochronous audio frame. Alpha.8 only suppressed unsolicited notifications — this fix suppresses all HID responses during streaming.

Trade-off: Dock display may freeze during playback (shows last known track info). Clean audio is the priority; display updates resume when streaming pauses/stops.

Also adds source mode diagnostics to the USB-DAC debug screen.