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/poller_test.go

@@ -623,9 +623,9 @@ func TestFetchableSeriesURL(t *testing.T) {
 		{"demonic https", "demonic", "https://demonicscans.org/manga/X", true},
 		{"comix https", "comix", "https://comix.to/title/n8we-dungeons-and-crayons", true},
 		{"kagane on its own host", "kagane", "https://kagane.to/series/019f84bc-9ba0-7ed9-86f5-8b905ec7c28b", true},
-		// The pins added in #94 cover the three plain-TLS Sites too: a
-		// client-supplied series_url must not aim a fetcher at a lookalike
-		// host, even when the fetcher is only an HTTP GET.
+		// The three plain-TLS Sites are pinned too: a client-supplied
+		// series_url must not aim a fetcher at a lookalike host, even when
+		// the fetcher is only an HTTP GET.
 		{"asura on a foreign host", "asura", "https://asurascans.com.evil.example/comics/x", false},
 		{"asura on the dead old domain", "asura", "https://asuracomic.net/comics/x", false},
 		{"demonic on a lookalike host", "demonic", "https://demonicscans.org.evil.example/manga/X", false},