Prober: claim ahead of the lanes, not when they run dry

The next claim started at half a batch of four pipeline rounds a lane,
3-6 s of work, while a claim takes 1.5-4 s and more; at 20-24 lanes the
lanes waited for candidates up to 19% of their time. queueLevels: the
claim starts below eight rounds of every lane's pipeline (~13 s) and
fills the queue to twice that. Taken candidates go back to waiting at
start-up as before, so a bigger queue costs nothing on a restart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/coordinator/prober_queue.spec.ts

@@ -0,0 +1,16 @@
+import { test } from '@japa/runner'
+import { queueLevels } from '#services/prober'
+
+test.group('Prober / queue levels', () => {
+  test('the next claim starts with eight pipeline rounds of every lane still queued, and fills to twice that', ({
+    assert,
+  }) => {
+    assert.deepEqual(queueLevels(22, 8, 400), { low: 1408, target: 2816 })
+    assert.deepEqual(queueLevels(7, 8, 400), { low: 448, target: 896 })
+  })
+
+  test('few lanes: at least the configured batch', ({ assert }) => {
+    assert.deepEqual(queueLevels(2, 8, 400), { low: 400, target: 800 })
+    assert.deepEqual(queueLevels(0, 8, 400), { low: 400, target: 800 })
+  })
+})