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>

commands/coordinator_run.ts

@@ -117,8 +117,11 @@ export default class CoordinatorRun extends BaseCommand {
         }
         if (main && Date.now() - tailedAt >= coordinatorConfig.tail.everySeconds * 1000) {
           tailedAt = Date.now()
-          const { groups, ranges } = await Planner.planTail()
-          if (ranges > 0) this.logger.info(`Tail: ${ranges} ranges for ${groups} groups.`)
+          const { groups, ranges, extended } = await Planner.planTail()
+          if (ranges + extended > 0)
+            this.logger.info(
+              `Tail: ${ranges} ranges added, ${extended} grown, for ${groups} groups.`
+            )
         }
       } catch (error: any) {
         this.logger.error(`Coordinator round failed: ${error.message}`)