Restore two review findings against spec claims 'the Poll keeps its own
Cover policy' and 'pinning is the only behavioural change': prefetchCover
now also runs on ticks where the page fetch itself fails (the heal is
independent of the page read, and its source may answer while the origin
does not), and the no-chapter log surfaces the fetched body length again
via a BodyLen fact on seriesRead. Browser dispatch iterates the sorted
browser site list so its outcome cannot depend on map order.
Extract gate, route, fetch and both parses into readSeriesPage, which
returns facts only; checkOne and acquire keep their scheduling, stamp
order and persistence policies. The poller also stops logging the fetched
body length, which only made sense while the body lived at the call site.
sulthan