Fix lossy slug-to-name/name-to-slug round-tripping

We convert spaces to periods when slugifying, which means we have no way to differentiate between periods which are periods and periods which are spaces when make the reverse conversion.

Locate the slug-to-name and name-to-slug conversion entry points and inspect any existing tests covering them. Determine a reversible representation for spaces and literal periods, then add coverage showing that both conversions round-trip without ambiguity.

Written by the indexing model from the issue text.