feat(editor): 标签编辑器保活，切换立即呈现（docs/18）

最近用过的 6 个标签保持挂载、隐藏而非销毁，切回零解析零序列化、各窗格
保留滚动位置；超限窗格卸载走最终回写。引用改为按标签注册表 + 活动变化
effect 统一指向；外部重载 epoch 按标签粒度、抑制闸门对所有标签生效。
EditorPane 新增就绪状态：首次打开大文件显示「正在打开文档…」加载态。
打印守卫：只出活动窗格。vitest 87 通过 + 2 跳过（新增 pane-loading 1 例）。

tests/pane-loading.test.tsx

@@ -0,0 +1,50 @@
+// 编辑器窗格加载状态：编辑器创建完成前显示「正在打开文档…」，就绪后撤下并回传 API。
+import { describe, it, expect, vi } from 'vitest'
+import { createRoot } from 'react-dom/client'
+import { act } from 'react-dom/test-utils'
+
+let resolveCreate: (() => void) | null = null
+const fakeEditor = {
+  create: () => new Promise<void>((resolve) => { resolveCreate = resolve }),
+  destroy: () => Promise.resolve(),
+  action: (fn: (ctx: unknown) => unknown) => fn({ get: () => { throw new Error('no view in test') } }),
+}
+
+vi.mock('../src/editor/editor', () => ({
+  createMarkdownEditor: () => fakeEditor,
+  getMarkdown: () => '',
+}))
+
+import EditorPane from '../src/components/EditorPane'
+
+describe('EditorPane 加载状态', () => {
+  it('创建未完成时显示加载提示，完成后撤下并回传就绪 API', async () => {
+    const container = document.createElement('div')
+    document.body.appendChild(container)
+    const root = createRoot(container)
+    const readyApis: unknown[] = []
+    await act(async () => {
+      root.render(
+        <EditorPane
+          tabId="t1"
+          initialMarkdown="# 大文档"
+          onMarkdown={() => undefined}
+          onCursor={() => undefined}
+          onReady={(api) => readyApis.push(api)}
+          focusMode={false}
+          typewriter={false}
+        />,
+      )
+    })
+    expect(container.textContent).toContain('正在打开文档')
+    expect(readyApis).toHaveLength(0)
+
+    await act(async () => { resolveCreate?.() })
+    expect(container.textContent).not.toContain('正在打开文档')
+    expect(readyApis).toHaveLength(1)
+    expect((readyApis[0] as { editor: unknown }).editor).toBe(fakeEditor)
+
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+})