fix: document apply ordering invariant; visible indeterminate checkbox + test

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

web/src/components/DiffView.test.tsx

@@ -131,3 +131,25 @@ test("select-all header checkbox calls onToggleAllUpdates(true) when clicked whi
   await user.click(screen.getByRole("checkbox", { name: /выбрать все.*updates/i }))
   expect(onToggleAllUpdates).toHaveBeenCalledWith(true)
 })
+
+test("select-all header checkbox is indeterminate when only some update rows are selected", () => {
+  const csWithMultipleUpdates: ChangesetResponse = {
+    updates: [
+      { key: "A www.example.com.", kind: "update", type: "A", name: "www.example.com.", desired: ["1.1.1.1"], actual: ["9.9.9.9"], readOnly: false },
+      { key: "A api.example.com.", kind: "update", type: "A", name: "api.example.com.", desired: ["1.1.1.2"], actual: ["9.9.9.8"], readOnly: false },
+      { key: "A cdn.example.com.", kind: "update", type: "A", name: "cdn.example.com.", desired: ["1.1.1.3"], actual: ["9.9.9.7"], readOnly: false },
+    ],
+    prunes: [],
+    readOnly: [],
+    inSyncCount: 0,
+  }
+  renderDiff({
+    changeset: csWithMultipleUpdates,
+    // Partial selection: one of three keys — neither all nor none — is what
+    // must drive the header checkbox into the indeterminate ("mixed") state.
+    selectedUpdates: new Set(["A www.example.com."]),
+  })
+
+  const selectAll = screen.getByRole("checkbox", { name: /выбрать все.*updates/i })
+  expect(selectAll).toHaveAttribute("aria-checked", "mixed")
+})