fix: deterministic disk tie-breaking and stronger pre-existing task test

- Sort available disks by NodeID then DiskID before scoring so
  destination selection is deterministic when two disks score equally
- Add task count bounds assertion to SkipsPreExistingPendingTasks test:
  with 15 of 20 volumes already having pending tasks, at most 5 new
  tasks should be created and at least 1 (imbalance still exists)

weed/worker/tasks/balance/detection.go

@@ -377,6 +377,14 @@ func planBalanceDestination(activeTopology *topology.ActiveTopology, selectedVol
 		return nil, fmt.Errorf("no available disks for balance operation")
 	}
 
+	// Sort available disks by NodeID then DiskID for deterministic tie-breaking
+	sort.Slice(availableDisks, func(i, j int) bool {
+		if availableDisks[i].NodeID != availableDisks[j].NodeID {
+			return availableDisks[i].NodeID < availableDisks[j].NodeID
+		}
+		return availableDisks[i].DiskID < availableDisks[j].DiskID
+	})
+
 	// Find the best destination disk based on balance criteria
 	var bestDisk *topology.DiskInfo
 	bestScore := math.Inf(-1)

weed/worker/tasks/balance/detection_test.go

@@ -576,6 +576,15 @@ func TestDetection_SkipsPreExistingPendingTasks(t *testing.T) {
 		}
 	}
 
+	// With 15 of 20 volumes on node-a already having pending tasks, only
+	// volumes 16-20 are eligible. Detection should produce at most 5 new tasks.
+	if len(tasks) > 5 {
+		t.Errorf("Expected at most 5 new tasks (only 5 eligible volumes remain), got %d", len(tasks))
+	}
+	if len(tasks) == 0 {
+		t.Errorf("Expected at least 1 new task since imbalance still exists with actual volume counts")
+	}
+
 	assertNoDuplicateVolumes(t, tasks)
 }