Docs: glossary and stale notes catch up with the finished-Series cutover (#161)

CONTEXT.md

@@ -155,6 +155,16 @@ Series (see `series.finished_at`), owned by the owner and stamped once, and ever
 on a finished Series is archived.
 _Avoid_: state, status (as a domain word), list
 
+**Finished Series**:
+A Series the owner has marked finished, stamped once in `series.finished_at`
+(epoch ms, zero means not finished). The owner is its only writer — no
+adapter, no Reader, no Poll can set it — and a Forced Poll reads a finished
+Series once for that pass and never clears the flag. It is a fact about the
+Series, not a Bookmark bucket: every Bookmark on a finished Series is
+archived, the Lane stops polling it (the due gate reads `finished_at = 0`),
+and Readers see a label and nothing more.
+_Avoid_: completed, done, dropped, shelved (that is Archived), ended
+
 **Favourite**:
 A reader's manual pin on a Bookmark. Orthogonal to the Lifecycle bucket, and never a
 reason to reorder the list.