phase: 20.49 COMPLETE; 20.50 Flow+String minimal reps; 20.51 selfhost v0/v1 minimal (Option A/B); hv1-inline binop/unop/copy; docs + run_all + CURRENT_TASK -> 21.0

apps/tests/concat-string-plugin/add_plugin_strings.hako

@@ -0,0 +1,9 @@
+static box Main {
+    main() {
+        local a = new StringBox("12")
+        local b = new StringBox("30")
+        local r = a + b
+        print(r)
+        return 0
+    }
+}