Sorting starts as descending by default!

This less-than-elegant inclusion of the warning in the computed
property is necessary because the mixing-in happens as the
application is initialised, so having it outside of the computed
property causes it to unhelpfully with no context.

This seems unhelpful to have printed repeatedly in tests.

These fields only differed by the inclusion of draining,
and the existing compositeStatus was only being used in
one place.

This removes an assertion that was unreliable with
certain random circumstances; it checked that the
composite status was equal to the node status, but that
wasn’t true for draining nodes. It felt okay to remove
because testing a generalised replacement would
necessitate reproducing the entire computed property and
there’s already an assertion that tests the specific
“ineligible” composite status.