The SweepError type-level rustdoc still read "A structural fault constructing a
`GridSpace`" though the enum now also gates `RandomSpace::new` (the three
random-only variants are each documented individually, but the type summary was
stale). Broaden it to name both spaces and group the grid vs. random faults.
Doc-only, behaviour-preserving — the same low-grade doc-debt class the 0049
audit fixed inline for the module doc. Surfaced by the cycle-0049 fieldtest.
refs #52
Brummel
be8d267019