fix: harden single-machine load handling

loadtest/im-pressure/model_test.go

@@ -24,6 +24,19 @@ func TestLoadConfigAppliesBoundedHistoryDefaults(t *testing.T) {
 	}
 }
 
+func TestSingleMachineExampleUsesSixtyPercentCapacity(t *testing.T) {
+	cfg, err := loadConfig("config.single-machine.example.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.UsersPerMachine != 600 || cfg.CloseRamp != "60s" {
+		t.Fatalf("unexpected single-machine capacity: users=%d close_ramp=%s", cfg.UsersPerMachine, cfg.CloseRamp)
+	}
+	if len(cfg.Endpoints["a"]) != 1 || len(cfg.Endpoints["b"]) != 1 {
+		t.Fatalf("single-machine endpoints must define both logical agents: %+v", cfg.Endpoints)
+	}
+}
+
 func TestNewManifestDistributesUsersAndEndpoints(t *testing.T) {
 	cfg := &Config{Tenant: "006", UsersPerMachine: 3, Endpoints: map[string][]string{
 		"a": {"ws://a1", "ws://a2"}, "b": {"ws://b1"},