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>

tests/unit/coordinator/tail.spec.ts

@@ -1,5 +1,5 @@
 import { test } from '@japa/runner'
-import { tailRanges, wildmatOf } from '#services/planner'
+import { tailMerges, tailRanges, wildmatOf } from '#services/planner'
 
 const options = { rangeArticles: 1000, minArticles: 100, maxDelayMinutes: 15, priority: -1 }
 const now = new Date('2026-10-09T10:00:00Z')
@@ -7,7 +7,7 @@ const minutesAgo = (m: number) => new Date(now.getTime() - m * 60_000)
 
 test.group('coordinator / live tail', () => {
   test('a chunk of new articles, or a long enough wait, makes tail ranges', ({ assert }) => {
-    const rows = tailRanges(
+    const { rows, extended } = tailRanges(
       [
         // Enough new articles: planned at once, cut at rangeArticles.
         { id: 1, plannedTo: 5000, plannedAt: minutesAgo(2), high: 7500 },
@@ -32,6 +32,94 @@ test.group('coordinator / live tail', () => {
         [3, 101, 103, 101, -1],
       ]
     )
+    assert.deepEqual(extended, [])
+  })
+
+  test('an open last range grows up to rangeArticles before rows are added', ({ assert }) => {
+    const { rows, extended } = tailRanges(
+      [
+        // Room in the open range for everything new.
+        {
+          id: 1,
+          plannedTo: 5300,
+          plannedAt: minutesAgo(20),
+          high: 5350,
+          open: { id: 11, start: 5001 },
+        },
+        // The open range fills up, the rest is cut as rows.
+        {
+          id: 2,
+          plannedTo: 5300,
+          plannedAt: minutesAgo(2),
+          high: 7500,
+          open: { id: 12, start: 5001 },
+        },
+        // An open range already full: rows only.
+        {
+          id: 3,
+          plannedTo: 6000,
+          plannedAt: minutesAgo(2),
+          high: 6500,
+          open: { id: 13, start: 5001 },
+        },
+        // Not open (taken, spooled, another priority): rows.
+        { id: 4, plannedTo: 5300, plannedAt: minutesAgo(20), high: 5350, open: null },
+      ],
+      now,
+      options
+    )
+    assert.deepEqual(
+      extended.map((e) => [e.id, e.group_id, e.from, e.end_id]),
+      [
+        [11, 1, 5301, 5350],
+        [12, 2, 5301, 6000],
+      ]
+    )
+    assert.deepEqual(
+      rows.map((r) => [r.group_id, r.start_id, r.end_id]),
+      [
+        [2, 6001, 7000],
+        [2, 7001, 7500],
+        [3, 6001, 6500],
+        [4, 5301, 5350],
+      ]
+    )
+  })
+
+  test('adjacent ranges of a group and priority pack into rangeArticles; gaps and other priorities break a run', ({
+    assert,
+  }) => {
+    const r = (id: number, group_id: number, start_id: number, end_id: number, priority = -3) => ({
+      id,
+      group_id,
+      start_id,
+      end_id,
+      priority,
+    })
+    const merges = tailMerges(
+      [
+        // 1–300 + 301–600 + 601–900 fill 1000 with 901–1000; 1001–1100 starts the next run alone.
+        r(1, 1, 1, 300),
+        r(2, 1, 301, 600),
+        r(3, 1, 601, 900),
+        r(4, 1, 901, 1000),
+        r(5, 1, 1001, 1100),
+        // A gap, then two adjacent at another priority.
+        r(6, 1, 1200, 1250),
+        r(7, 1, 1251, 1300, -1),
+        r(8, 1, 1301, 1400, -1),
+        // Another group; one that would overflow stays alone.
+        r(9, 2, 1, 900),
+        r(10, 2, 901, 1100),
+        r(11, 2, 1101, 1150),
+      ],
+      1000
+    )
+    assert.deepEqual(merges, [
+      { id: 1, end_id: 1000, drop: [2, 3, 4] },
+      { id: 7, end_id: 1400, drop: [8] },
+      { id: 10, end_id: 1150, drop: [11] },
+    ])
   })
 
   test('LIST ACTIVE patterns by the first two name parts', ({ assert }) => {