feat: add preferred debug port planning

internal/platform/config/store_test.go

@@ -42,6 +42,50 @@ func TestStoreAddsDefaultRemoteDebugPortForExistingConfig(t *testing.T) {
 	}
 }
 
+func TestStoreAssignsPreferredPortsForLegacyInstancesInStableOrder(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.json")
+	contents := `{"version":1,"settings":{"remoteDebugStartPort":9777},"instances":[{"id":"first","launch":{"Kind":"chrome","UserDataDir":"C:\\profiles\\first"}},{"id":"second","launch":{"Kind":"edge","UserDataDir":"C:\\profiles\\second"}}]}`
+	if err := os.WriteFile(path, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	store, err := New(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := store.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Instances[0].PreferredRemoteDebugPort != 9777 || got.Instances[1].PreferredRemoteDebugPort != 9778 {
+		t.Fatalf("legacy preferred ports = %#v", got.Instances)
+	}
+	if err := store.Save(got); err != nil {
+		t.Fatal(err)
+	}
+	reloaded, err := store.Load()
+	if err != nil || reloaded.Instances[0].PreferredRemoteDebugPort != 9777 || reloaded.Instances[1].PreferredRemoteDebugPort != 9778 {
+		t.Fatalf("persisted preferred ports = %#v, error = %v", reloaded.Instances, err)
+	}
+}
+
+func TestStoreRejectsInvalidOrDuplicatePreferredPorts(t *testing.T) {
+	store, err := New(filepath.Join(t.TempDir(), "config.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	duplicate := File{Instances: []Instance{
+		{ID: "one", PreferredRemoteDebugPort: 9666},
+		{ID: "two", PreferredRemoteDebugPort: 9666},
+	}}
+	if err := store.Save(duplicate); err == nil {
+		t.Fatal("duplicate preferred port was accepted")
+	}
+	invalid := File{Instances: []Instance{{ID: "one", PreferredRemoteDebugPort: domain.MinRemoteDebugPort - 1}}}
+	if err := store.Save(invalid); err == nil {
+		t.Fatal("invalid preferred port was accepted")
+	}
+}
+
 func TestStoreMissingFileReturnsDefaults(t *testing.T) {
 	store, err := New(filepath.Join(t.TempDir(), "config.json"))
 	if err != nil {