refactor: modernize Go with strings.SplitSeq and maps.Copy

Replace strings.Split loops with strings.SplitSeq iterators in the CSV
parsers (reality_scan and the scale-test helpers) and swap a manual map
copy for maps.Copy in the MTProto traffic collector. No behavior change;
these are the fixes the modernize analyzer reports.

internal/sub/sub_scale_test.go

@@ -54,7 +54,7 @@ func scaleSubSizes(t *testing.T, def ...int) []int {
 		return def
 	}
 	var out []int
-	for _, part := range strings.Split(raw, ",") {
+	for part := range strings.SplitSeq(raw, ",") {
 		part = strings.TrimSpace(part)
 		if part == "" {
 			continue