docs: custom records and import-as-sync invariants

Document the two Task 1-9 features in CLAUDE.md invariants and README
features. Also closes two review gaps: a Go test proving importResponse
marshals created/removed as [] (never null) on a no-op import, and a
frontend test-race fix in DomainsPage.test.tsx where the import button
was clicked before its disabled state cleared.

internal/api/tenant_test.go

@@ -531,6 +531,35 @@ func TestImportZones_BadAccountUUID(t *testing.T) {
 	}
 }
 
+// TestImportResponseEmptyMarshalsToArrays guards the same белый-экран class
+// of bug as TestChangesetResponseEmptyMarshalsToArrays (api_test.go), but for
+// the import endpoint: a sync that created and removed nothing (empty zone
+// list from the provider) must still marshal created/removed as [], not
+// null — the frontend's .length/.map calls on the response would otherwise
+// crash right after a no-op import.
+func TestImportResponseEmptyMarshalsToArrays(t *testing.T) {
+	a, ts := newTenantTestAPI()
+	accID := uuid.New()
+	ts.accounts = []store.Account{{ID: accID, Provider: "selectel", SecretEnc: "ENC(token)"}}
+	a.Reg = &mockRegistry{zones: nil}
+	router := NewRouter(a)
+
+	req := requestWithSessionCookie(http.MethodPost, "/api/v1/projects/"+testPID+"/accounts/"+accID.String()+"/import", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusCreated {
+		t.Fatalf("status %d body %s", w.Code, w.Body.String())
+	}
+	body := w.Body.String()
+	if !strings.Contains(body, `"created":[]`) {
+		t.Fatalf("expected \"created\":[] in %s", body)
+	}
+	if !strings.Contains(body, `"removed":[]`) {
+		t.Fatalf("expected \"removed\":[] in %s", body)
+	}
+}
+
 func TestCreateDomain_BadProjectUUID(t *testing.T) {
 	a, _ := newTenantTestAPI()
 	router := NewRouter(a)