The mask is the writer's and is stamped once, at the moment the record's
class is known for certain; every later reader is spared re-deriving it. It
means nothing to this side — which bits are which class is a vocabulary's
business, and a kernel that interpreted one would be inventing a vocabulary.
0 is "no class", which is what every record written before the field existed
reads as, so a caller that does not classify is not writing a special value.
A store whose image is the legacy packed layout has no field to put a mask in
and drops it: that layout cannot carry a class, and saying otherwise would be
a lie about the bytes on disk.
The field is appended, so sizeof(cube_args) grows from 80 to 88 — still
distinct from the other three argument blocks, which is what the size-first
dispatch depends on.
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