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.

web/src/pages/DomainsPage.test.tsx

@@ -88,6 +88,8 @@ test("после импорта показывает, сколько зон со
   const user = userEvent.setup()
   renderPage()
 
+  await screen.findByText("example.com.")
+
   await user.click(await screen.findByRole("button", { name: /импортировать зоны/i }))
 
   expect(await screen.findByText(/создано 1, удалено 1/i)).toBeInTheDocument()