Also, mark not just curves, but also points and normals derived from
construction requests as construction.
Also, don't always mark arc center point as construction just to
exclude it from chord tolerance bounding box calculation; instead,
special-case it there.
This is a bit slow, but more importantly creates a race condition
where e.g. a failing "normal" test will be overwritten by
a succeeding "normal_migrate_from_v20".
This shouldn't ever be a problem since saving dumps the entire
internal state, or is supposed to, at least.
