How to represent bits with a required zero value?

Pixel formats have padding when channel bit ranges don't cover all bits (e.g. 10-bit formats with 3 channels may have 2 bits of padding). For some formats the padding may contain arbitrary/undefined contents, but for some others the padding must be filled with zeroes. Examples include `DRM_FORMAT_Y210` and `AV_PIX_FMT_P010`.

How would one represent such bits with KDFS?

KDFS defines constant virtual bits, but I haven't found a way to use these to represent such required zero values.

No file or test is named. Start by reviewing how KDFS defines constant virtual bits, then compare the representations needed for DRM_FORMAT_Y210 and AV_PIX_FMT_P010. Done means the specification has an unambiguous way to represent padding bits that must contain zeroes.

Written by the indexing model from the issue text.