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.

docs/adr/0009-a-site-answers-questions-its-own-way.md

@@ -9,8 +9,10 @@ Per-Site knowledge lives in one registry in `backend/internal/latest/sites.go`,
 keyed by the stored site string. An entry answers a fixed set of questions: the
 hostname a `series_url` must carry, how to find the Latest Chapter in a body,
 how to find the Cover address in a body, and — for a Site behind a JavaScript
-challenge — how to read its payload from a cleared tab and how to tell that the
-payload arrived.
+challenge — how to read its payload from a cleared tab, how to tell that the
+payload arrived, and whether the plain-TLS fetcher may take over when no
+browser is configured (`Fallback`; kagane never falls back, novelfull does,
+each on measured evidence).
 
 The question set does not grow to accommodate one Site. When a Site needs
 something the set cannot express, that Site gets an optional override and