One node of a DownloadResult.version_history chain — a single known revision of the
series, as the plugin's site reports it.
source: string
This revision's source URL, already normalized through this plugin's own
canonicalizeSource (see DownloadResult.version_history's docs — an
un-normalized value silently fails to match a stored source: tag).
posted_at: string
ISO 8601 timestamp the site reports for this revision. The plugin converts from whatever the
site's native format is (e-hentai's gdata reports posted as a unix-seconds string). Sole
determiner of revision order: there is deliberately no separate "this one is current" flag,
since a second source of truth could contradict the sort and leave the host no way to know
which to believe.