New canvas-header Mic toggle, shown for boards whose ProBoardDef declares
builtInMicrophone (their bridge implements setMicrophoneSource). Capture
runs at the AudioContext's native rate, is decimated to 16 kHz and ring-
buffered (2 s, drop-oldest on overflow, silence on underflow - the engine
paces pulls in simulated cycles, up to 4x real time during boot boost).
No auto-start: with the toggle off the bridge's 440 Hz test tone keeps
mic sketches alive, so taking the user's microphone stays an explicit
click. The source is re-applied on every run start to survive bridge
rebuilds.
e96dae794a