disk-load retires the readings of a module whose key went while its watch was down (review round 2)

A key deleted while disk-load was not watching (a restart, a reboot, a machine asleep) never
arrives as a delete. Once the watch has handed over every current key, the readings of modules
with no key are retired once; an empty set is skipped and said, since the engine has not written.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/disk-load/cmd/disk-load/data_test.go

@@ -134,6 +134,37 @@ func TestTheItemsComeFromTheEnginesKeysAndARoundSaysHowFarItGot(t *testing.T) {
 	if measured["/srv/web2"] != before || round().Items != 2 {
 		t.Errorf("an item whose key was deleted was measured again: %d times, round %+v", measured["/srv/web2"]-before, round())
 	}
+
+	// A restart: oldie's key was deleted while the watch was down, so no delete arrives. Once the watch handed over its
+	// keys, oldie's reading is retired and pg's kept; an empty set retires nothing (the engine has not written yet).
+	_ = readings.Put("ace.oldie.x", health.Reading{V: health.ReadingVersion, Node: "ace",
+		PlacedDataItem: catalogue.PlacedDataItem{Module: "oldie", ID: "x", Path: "/srv/oldie"}})
+	for _, restart := range []struct {
+		name    string
+		keys    []stdio.StateChange
+		retired bool
+	}{
+		{"no key yet", nil, false},
+		{"pg's key alone", []stdio.StateChange{{Key: key("pg"), Op: stdio.OpPut, Value: value("pg", pg, gone), Current: true}}, true},
+	} {
+		w2 := &Watched{Node: "ace"}
+		said = nil
+		k2 := &Keeper{Node: "ace", Items: w2.All, Store: readings, Say: k.Say, Measurer: k.Measurer}
+		for _, ch := range restart.keys {
+			if _, _, err := w2.Take(ch); err != nil {
+				t.Fatal(err)
+			}
+		}
+		w2.Ready()
+		k2.RetireUnkeyed(context.Background())
+		oldie, pgData := readingOf(readings, "ace.oldie.x"), readingOf(readings, "ace.pg.data")
+		if oldie == nil || pgData == nil || oldie.RetiredAt.IsZero() == restart.retired || !pgData.RetiredAt.IsZero() {
+			t.Errorf("%s: oldie %+v, pg %+v; oldie retired: want %v, pg never", restart.name, oldie, pgData, restart.retired)
+		}
+		if !restart.retired && !strings.Contains(strings.Join(said, "\n"), "holds no key") {
+			t.Errorf("%s: the skip was not said: %v", restart.name, said)
+		}
+	}
 }
 
 func readingOf(s *memStore, key string) *health.Reading {