The tail planner grows a group's open range; ranges:merge-tail packs the slivers

A round added a row a group whenever 100k articles or 15 minutes had passed,
tuned for a tail leased within minutes; with nothing leasing the tail the rows
piled up (127k planned tail ranges of a median 14k articles, 97% adjacent). Now
a round grows the group's last range when it is a planned, unspooled, untaken
row at the tail's priority, up to RANGE_ARTICLES, and only then adds rows; a
range taken meanwhile gets the slice as a row. ranges:merge-tail (--dry-run)
packs adjacent planned, uncaptured, unspooled, untaken tail ranges of a group
and priority into ranges of RANGE_ARTICLES under the lease lock.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

REBUILD.md

@@ -62,7 +62,12 @@ the cap is shared by the node's processes).
 
 Also built (2026-10-09): the live tail (new articles planned every 2
 minutes from one LIST ACTIVE per name prefix, at priority −1: leased and
-fetched before history); the catalog API, Newznab (`/api`) and the web
+fetched before history; since 2026-10-27 a round grows the group's open
+tail range, a planned one at the tail's priority nothing spooled or
+took, up to RANGE_ARTICLES before it adds rows, as a tail nothing leased
+had become a sliver a group a round, 127k rows of a median 14k articles;
+`ranges:merge-tail` packs what such rounds left); the catalog API,
+Newznab (`/api`) and the web
 interface, from voyager 1 (the current build), with categories from the
 name; voyager 1's 29 learned garbage signatures (`rules:signatures`,
 `database/data/voyager1_signatures.tsv`).