Portable Device.functions() omits video (FN_VIDEO exists in C)

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What is missing

_usbif.FN_VIDEO (bit 32) is built into the C module and examples/usbif_webcam.py uses it. The portable costume API does not know the name:

# lib/usbif/native_usb.py
_BITS = {"cdc": 1, "msc": 2, "uac": 4, "midi": 8, "hid": 16}
# no "uvc" / "video"

So usbif.auto.device().functions("cdc", "uvc") raises, and the webcam example has to poke _usbif directly.

Why it matters

Costume selection is supposed to be the portable surface. Every other device class is reachable through it; video is the odd one out, which means applications that want a webcam costume cannot stay on the portable API.

Fix shape (for whoever lands it)

Add "uvc": 32 (or "video": 32, matching the C name — pick one and document it) to NativeDevice._BITS and to Device.FUNCTIONS, and extend the costume self-test matrix.