feat: add controlled workbench sidebar resize

docs/reference/cloud-workbench.md

@@ -40,6 +40,17 @@ The default workbench follows the #99 VS Code-style structure:
 Unavailable or future controls must be disabled with blocker context. The
 frontend must not imply generic direct hardware control.
 
+The left resource explorer is a controlled resizable column on desktop-class
+viewports. Its width defaults to `292px`, clamps inside a bounded range
+(`220px` to `420px` before viewport-specific clamping), and is changed through
+the `#explorer-resize` separator. The separator has a Chinese accessible name,
+supports pointer drag plus Arrow/Page/Home/End keyboard adjustment, and stores
+only the clamped width in versioned `localStorage` key
+`hwlab.workbench.layout.v1`. Collapsing the resource tree hides the column but
+keeps the saved width so expanding restores it. At `<=860px`, the resize handle
+is removed from the focus/hit-target flow and the mobile drawer/collapse flow
+owns the layout.
+
 The `panel-wiring` / `hwlab-patch-panel` view must use a two-column long table
 instead of an eight-column wide table. The first table row/header names the two
 connected devices, for example `res_boxsimu_1` and `res_boxsimu_2`; following
@@ -121,6 +132,12 @@ runner 入口。它使用 Playwright 覆盖 `1366x768`、`1024x768` 和 `390x844
 点击与 `document.elementsFromPoint()`；不得使用 `force: true` 或纯 DOM
 dispatch 绕过用户命中。
 
+Layout smoke also covers #278 resize behavior. Desktop and narrow desktop runs
+drag `#explorer-resize`, exercise Arrow/Home/End keyboard changes, assert
+min/max/clamp and versioned storage restore, then collapse and expand the
+resource tree to prove width state is retained. The mobile run asserts the
+handle is not displayed or focusable and that no horizontal overflow appears.
+
 常用命令：
 
 ```sh
@@ -171,6 +188,8 @@ blockers/failures：`viewport`、`selector`、`failureType`、summary 和 artifa
   Playwright layout and overlap guardrail.
 - [pikasTech/HWLAB#276](https://github.com/pikasTech/HWLAB/issues/276):
   patch-panel wiring panel two-column long-table feedback.
+- [pikasTech/HWLAB#278](https://github.com/pikasTech/HWLAB/issues/278):
+  resource explorer collapse and controlled resize feedback.
 - [code-agent-chat-readiness.md](code-agent-chat-readiness.md): Code Agent
   chat 同源 readiness 和 provider credential blocker 边界。
 - [docs/cloud-web-workbench.md](../cloud-web-workbench.md): detailed frontend