CoverageSpanDto
One inclusive [start, end] coverage range. Mirrors Codex's NumericSpan
(single values are start == end) so a release plugin can ingest the
volumeCoverage / chapterCoverage lists verbatim.
endnumber<double>required
startnumber<double>required
CoverageSpanDto
{
"end": 0,
"start": 0
}