swapGuidEndianness

Interpret 16 bytes as a little-endian (Microsoft) GUID, as PlayReady stores Key IDs.

The first three GUID components are byte-reversed relative to the big-endian form used by cenc:default_KID and Widevine. The swap is its own inverse, so this also converts back. Note pywidevine does not swap (pssh.py:276) and therefore reports PlayReady KIDs that disagree with the manifest's own cenc:default_KID; the manifest is the authority, so ktvine diverges here deliberately.