fix(robot): make a dot between two handle words rare, prefer underscore

A handle joining two meaningful words with a dot ("Тихий.Воин", "Hidden.Hunter")
reads as machine-generated — people use "_" there, a dot only rarely. assembleHandle
now picks "_" about nine times in ten and "." only about one in ten for the
separator-joined form; the camelCase and bare-noun forms are unchanged.

backend/internal/robot/namevariety.go

@@ -140,20 +140,23 @@ func cyrillicNick() string {
 	return assembleHandle(form, noun.word)
 }
 
-// assembleHandle joins an adjective and noun into a handle: a bare noun, the two joined
-// by "_" or "." or camel-cased, optionally followed by a trailing number. It keeps at
-// most one separator so the result, the number aside, reads like a name a human could pick.
+// assembleHandle joins an adjective and noun into a handle: a bare noun, the two
+// camel-cased ("DarkWolf"), or joined by a separator. Between two meaningful words a human
+// almost always uses "_" — a "." there is a rare tell — so the separator is an underscore
+// far more often than a dot. An optional trailing number may follow.
 func assembleHandle(adj, noun string) string {
 	var base string
 	switch rand.IntN(4) {
 	case 0:
-		base = noun
+		base = noun // a bare noun
 	case 1:
-		base = adj + "_" + noun
-	case 2:
-		base = adj + "." + noun
+		base = adj + noun // camel-cased
 	default:
-		base = adj + noun
+		sep := "_"
+		if rand.IntN(10) == 0 {
+			sep = "." // a dot between two words is rare
+		}
+		base = adj + sep + noun
 	}
 	if rand.IntN(2) == 0 {
 		base += nickNumber()