Spec #136: finished becomes a Series fact, Lane gate rewritten on it (#157)

finished_at lands on series (0016 seeds it from the pre-flip finished
bookmarks, then flips those bookmarks to archived), the poll gate and
eligible count read the flag instead of a per-Reader vote, PUT rejects
the finished status like any unknown value, and the web UI drops the
Finished tab, badge and finish button. Novel merge rank is archived >
reading. Per-Reader disagreement (one Reader keeps a finished Series in
reading forever) is what the flag repairs; the cutover keeps polling
state unchanged for every Series.

CONTEXT.md

@@ -95,7 +95,7 @@ _Avoid_: run, cycle, tick, batch, poll history
 **Forced Poll**:
 A Poll the owner asks for by hand instead of waiting for the Series's turn. It jumps its
 Lane's queue and ignores every waiting rule — the rest between Polls, a Sighting standing
-in for a check, a Series only finished Readers hold — but never overrules a Site that is
+in for a check, a finished Series — but never overrules a Site that is
 refusing us, the Lane's spacing between fetches, or a Series with no page to fetch. Asked
 for by marking the Series, never by commanding the poller, so it happens on the Lane's
 next pass rather than at the moment of asking.
@@ -149,8 +149,10 @@ accent is permitted to signal.
 _Avoid_: unread, update available
 
 **Lifecycle bucket**:
-Which of three mutually exclusive states a Bookmark sits in — reading, archived, or
-finished. A Bookmark is in exactly one. Orthogonal to being a favourite.
+Which of the two states a Bookmark sits in — reading or archived. A Bookmark is in exactly
+one. Orthogonal to being a favourite. Finished is not a bucket: it is a fact about the
+Series (see `series.finished_at`), owned by the owner and stamped once, and every Bookmark
+on a finished Series is archived.
 _Avoid_: state, status (as a domain word), list
 
 **Favourite**: