fix: keep the legacy cover heal and no-chapter byte count (#94)

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.

backend/internal/latest/read.go

@@ -15,6 +15,9 @@ type seriesRead struct {
 	HasLatest bool
 	Cover     string
 	HasCover  bool
+	// BodyLen is the fetched body's length, surfaced because the no-chapter
+	// log uses it to tell a markup change from a body the size cap cut short.
+	BodyLen int
 }
 
 // errNotFetchable and errNoFetcher separate the gate and the route from fetch
@@ -51,5 +54,5 @@ func readSeriesPage(ctx context.Context, site, seriesURL string, browser, tls Fe
 	}
 	latest, hasLatest := latestChapterFrom(site, seriesURL, body)
 	cover, hasCover := coverFrom(site, seriesURL, body)
-	return seriesRead{Latest: latest, HasLatest: hasLatest, Cover: cover, HasCover: hasCover}, nil
+	return seriesRead{Latest: latest, HasLatest: hasLatest, Cover: cover, HasCover: hasCover, BodyLen: len(body)}, nil
 }