The phase 3 section named the key and left the scope to be inferred from the
fact that deviceid is one field. Spelling it out: DeviceId is a single value,
not a key per device type, and the pc-type decides which sync consumes it - so
CMM, Keyence, Genspect, wax-trace and part markers all read the same value, and
a family declared later through subordinatedevice_<pctype> is covered the day
it is declared.
Also records what the value REPLACES per family, which differs: for a CMM it
replaces cmmid.txt as identity only, for a part marker it replaces minting from
the hostname, and for the other three it replaces nothing because they never
had an identity at all.
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