fix(ui): move the onboarding takeover chrome into the step

The settings shell painted the onboarding overlay (opaque stage, mask,
#root inert) the moment a step was registered and not locally completed,
while every step still had to load its private join before deciding
whether to show — rendering null could not suppress the shell-owned
chrome. Every reload on the hero therefore flashed a full-screen opaque
layer (white in the light palette) for one settings/credential RPC
round-trip after the session list turned ready.

The chrome now belongs to the step: a new zero-cordis OnboardingSurface
primitive (ui-primitives) renders the body-portaled overlay/mask/stage
verbatim from the former SettingsRoot stylesheet and holds #root inert
for exactly its own lifetime. WelcomeNotice and DeepSeekOnboardingDialog
wrap only their visible branch in it, so their existing null branches
paint and block nothing by construction. SettingsRoot keeps the
coordinator unchanged but renders the elected step bare, and the
settings.onboarding contract now names the surface wrap as the
registrant's obligation.

The onboarding e2e gains a held-join reload scenario pinning that a
configured world never mounts the takeover chrome or inerts the app.

packages/client/ui-primitives/README.zh.md

@@ -2,7 +2,7 @@
 
 [English](README.md) | 中文
 
-纯 React 原子组件（零 cordis）：StateDot、ic_ds_* 图标、Button/Pill/Menu/Modal/Input、markdown 家族（MessageText/MarkdownText/JsonBlock）、只读 JsonTree 检查器、`useAnchoredMaxHeight` hook（把底部锚定的浮层高度收敛到锚点上方的视口空间，并在 resize、scroll 与调用方提供的依赖变化时重新测量）、TerminalBlock、DiffBlock、ReadBlock、SearchBlock，以及 WebBlock。契约：api-contracts v3 §8。
+纯 React 原子组件（零 cordis）：StateDot、ic_ds_* 图标、Button/Pill/Menu/Modal/Input、OnboardingSurface 首次使用接管层（portal 到 body 的遮罩加不透明展示层，在自身生命周期内保持 `#root` 为 `inert`）、markdown 家族（MessageText/MarkdownText/JsonBlock）、只读 JsonTree 检查器、`useAnchoredMaxHeight` hook（把底部锚定的浮层高度收敛到锚点上方的视口空间，并在 resize、scroll 与调用方提供的依赖变化时重新测量）、TerminalBlock、DiffBlock、ReadBlock、SearchBlock，以及 WebBlock。契约：api-contracts v3 §8。
 
 ## 悬浮卡片