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

src/client/center-column.ts

@@ -0,0 +1,22 @@
+/** Stable selector for the DSH AppFrame conversation slot. */
+const CENTER_COLUMN_SELECTOR = '#root [data-slot="conversation"]'
+
+/**
+ * Resolve the AppFrame center column while keeping the hot path query-free.
+ *
+ * Streaming chat output mutates `#root` at token cadence, so Sidebar's
+ * MutationObserver can call this once per animation frame. Once the center
+ * column has been found, its DOM identity is stable for the conversation
+ * lifecycle; checking `isConnected` is enough to reuse it without scanning
+ * the whole app tree again. Boot/HMR replacement still falls through to the
+ * document query as soon as the cached node is detached.
+ */
+export function resolveCenterColumn(
+  current: HTMLElement | null,
+  query: () => Element | null = () => document.querySelector(CENTER_COLUMN_SELECTOR),
+): HTMLElement | undefined {
+  if (current !== null && current.isConnected) return current
+
+  const col = query()?.parentElement
+  return col !== null && col !== undefined && col.isConnected ? col : undefined
+}