perf: tag the center column instead of a :has() selector; stop re-stringifying the editor doc per keystroke

The bottom-push rule anchored on #root :has(> [data-slot=conversation]):
a :has() match cache invalidates on every #root subtree mutation, and
the streaming chat mutates that subtree constantly — every layout-push
frame (the bottom drag writes --dsh-sidebar-height per frame) paid a
whole-document selector re-evaluation. The shell's locator (which
already holds the measured column node for the ResizeObserver) now
tags it with [data-dsh-center-col]; the rule evaluates once per node.
The tag retargets with the ref on boot swap / HMR node replacement and
is removed on unmount; the drag lane pins exactly-one-tagged-node and
its identity as the conversation slot wrapper's parent. The dead
[data-pane] branch (renamed away in alpha hosts) is gone.

TextEditor: the CodeMirror updateListener re-stringified the WHOLE
document into React state on every keystroke (O(docLength) per key,
re-rendering the shell per key). The listener now only flips the dirty
flag; the preview's draft is snapshotted from the live view when
entering preview (or on a content swap), and save() already reads the
live document.

docs/external-plugin-guide.md

@@ -752,7 +752,7 @@ ctx.effect(() =>
 - **面板表面**：右/底面板背景 = `var(--dsw-alias-bg-layer-1)`。**绝不消费 `--dsw-specific-sidebar-fill`**（宿主左导航专属，皮肤按左导航语义覆盖它，面板消费会失去填充）。换面板表面 = 覆写 `--dsw-alias-bg-layer-1`。
 - **终端/编辑器表面**：`effectiveTokenValue` 读 `--dsw-alias-bg-base`——`transparent` 与 alpha < 0.9 的半透明值回退不透明底色（文字不叠背景画，issue #90）；≥ 0.9 放行。
 - **根锚点**：宿主 div 带 `data-dsh-better-sidebar`（append 到 body）；其内**面板宿主层** `[data-dsh-panel-host]`（`fixed; inset:0; z-25; pointer-events:none`，v0.13.1+），面板/开关簇 absolute 定位，免疫中间层 transform 劫持；页面级 transform 触发 `data-dsh-panel-host-degraded` 降级。皮肤作用域覆盖限定在 `[data-dsh-better-sidebar]` 内。
-- **布局变量**（`<html>` 上，面板打开时有效）：`--dsh-sidebar-width` / `--dsh-sidebar-height`。宿主推挤 = `#root` margin/width calc + centerCol `margin-bottom`；锚点用**复合选择器双保险**（禁止 `nth-child`）：`#root [data-dsh-frame] > [data-pane="conversation"]` 与 `#root :has(> [data-slot="conversation"])`（两代命名同元素，`drag-layout.e2e.ts` 断言）。
+- **布局变量**（`<html>` 上，面板打开时有效）：`--dsh-sidebar-width` / `--dsh-sidebar-height`。宿主推挤 = `#root` margin/width calc + centerCol `margin-bottom`；锚点 = **JS 标注**（禁止 `nth-child`）：侧栏 shell 的定位器给测得的 centerCol 节点打 `[data-dsh-center-col]` 标签（`Sidebar.tsx` locate，节点更换/HMR 时随 ref 迁移），`layout.css` 用 `#root [data-dsh-center-col]` 选中（`drag-layout.e2e.ts` 断言恰一节点且为 `[data-slot="conversation"]` 的父级）。
 - **桌面信号与标题栏**（v0.14.1+ 四方案模型 `SidebarPrefs.titleBarScheme`，唯一决策点 `src/client/titlebar-strip.ts` 纯函数）：
   - 壳信号（只读，不自动触发修改）：URL `dsh-desktop-mode` / `dsh-desktop-platform` / 可选 `dsh-desktop-titlebar-inset`（0–120 clamp）。
   - **strip 取值链**：⓪ `web` 方案强制 0；① `navigator.windowControlsOverlay` 真实几何（`wco.ts` 订阅 `geometrychange`，**为 0 也权威**，`visible=false` 幽灵 API 视为缺失）；② URL inset；③ 壳预设 `stripFor`（仅 `preset`）；④ 手动 `titleBarStripPx`（仅 `custom`）；⑤ 0。驱动 `body[data-dsh-title-bar-compat]` + `--dsh-title-bar-strip`。