The sweep asked `app_label()` whether a package still needed a label, and
that accessor answers "what should be displayed" — so a definitive negative
(a package whose pm dump ran and carried no label) came back as None,
indistinguishable from "never asked". Every sweep therefore re-dumped every
label-less package, throwing away the 24 h cache the label path exists for,
and a second sweep cost a full dump round for the whole app list.
`_known_label(package) -> (known, label)` is the accessor that keeps the two
apart: known-and-empty means the device already answered, so the row falls
back to its package id with ZERO further round trips. A forced re-sweep now
issues only the two list commands.
Pinned by tests: a second sweep dumps nothing at all (negatives included),
and a package whose fetch FAILED (no output — the UNIT 0 rule) stays
uncached and is retried by the next sweep, recovering its label.
James Mackay
ff8dd1c454
