perf(sidebar): 避免已定位会话列时重复查询 DOM

- 缓存会话列仍连接时跳过 #root 全树 querySelector
- 保留 detached/HMR 回退定位与 1.5s safety net
- 补充 locator 行为测试覆盖缓存复用与节点替换

tests/center-column-locator.spec.ts

@@ -0,0 +1,53 @@
+// @vitest-environment jsdom
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { resolveCenterColumn } from '../src/client/center-column.ts'
+
+afterEach(() => {
+  document.body.innerHTML = ''
+})
+
+function connectedColumn(): { col: HTMLDivElement; slot: HTMLDivElement } {
+  const col = document.createElement('div')
+  const slot = document.createElement('div')
+  slot.dataset.slot = 'conversation'
+  col.append(slot)
+  document.body.append(col)
+  return { col, slot }
+}
+
+describe('center-column locator (issue #403)', () => {
+  it('reuses a connected cached column without querying the app tree', () => {
+    const { col } = connectedColumn()
+    const query = vi.fn<() => Element | null>(() => null)
+
+    expect(resolveCenterColumn(col, query)).toBe(col)
+    expect(query).not.toHaveBeenCalled()
+  })
+
+  it('queries and adopts the replacement after the cached column detaches', () => {
+    const stale = connectedColumn().col
+    stale.remove()
+    const { col: replacement, slot } = connectedColumn()
+    const query = vi.fn<() => Element | null>(() => slot)
+
+    expect(resolveCenterColumn(stale, query)).toBe(replacement)
+    expect(query).toHaveBeenCalledTimes(1)
+  })
+
+  it('queries when no column has been cached yet', () => {
+    const { col, slot } = connectedColumn()
+    const query = vi.fn<() => Element | null>(() => slot)
+
+    expect(resolveCenterColumn(null, query)).toBe(col)
+    expect(query).toHaveBeenCalledTimes(1)
+  })
+
+  it('rejects a missing or detached query result', () => {
+    expect(resolveCenterColumn(null, () => null)).toBeUndefined()
+
+    const detachedCol = document.createElement('div')
+    const detachedSlot = document.createElement('div')
+    detachedCol.append(detachedSlot)
+    expect(resolveCenterColumn(null, () => detachedSlot)).toBeUndefined()
+  })
+})