feat(files): tree rename (inline) + delete (confirmed) + submenu visual tether

资源管理器补全：文件树右键菜单新增「重命名」（行内编辑：预填+全选、
Enter/失焦提交、Esc 取消、IME 组合键守卫）与「删除」（Modal 确认，文件/
目录文案区分，永久删除——宿主无回收站）。根行（工作区根）不提供两项，
服务端 fs.rename/fs.remove 双保险拒绝；目标已存在 409；link-aware——
symlink 行只改/删链接本身（lstat 判定），fence 校验对解析后目标进行。

落定收尾：pruneTree 清旧前缀缓存 + 收起 stale expanded + 单层重载父目录；
tab 调解走 service（rename→updateTab 改路径改标题、delete→closeTab 关
受影响 tab，覆盖两侧分栏/底栏/浮动窗口，经 tree-mutations.ts）。

二级菜单视觉关系（layout.css，仍限 data-dsh-sidebar-submenu 作用域）：
父行在子菜单打开期间保持高亮（宿主已渲染 aria-expanded="true"），
卡片间距 10px→4px（悬停走廊同步收窄），翻转规则一并 4px。

i18n 新增 7 key × 20 语言文件（locales.spec 的 key-set-parity 门禁）。
测试：fs-operations 追加 12 例（rename/remove 全失败面）；新增
file-tree-rename-delete.spec.tsx 8 例（菜单可见性/重命名流/删除流/错误条）；
menu-flip.spec.tsx 契约扩展覆盖 tether 规则。设计文档
docs/plans/2026-09-04-file-tree-rename-delete-design.md。

src/client/layout.css

@@ -131,8 +131,31 @@ body[data-dsh-sidebar-dragging] #root [data-dsh-center-col],
  * closes on the other's outside pointerdown).
  *
  * No max-height/overflow scroll cap here on purpose: overflow clipping
- * would cut the submenu's ::before hover bridge (the 10px corridor that
- * keeps itemWrap's mouseleave from closing the card mid-crossing). */
+ * would cut the submenu's ::before hover bridge (the corridor that keeps
+ * itemWrap's mouseleave from closing the card mid-crossing). */
+
+/* Visual tether, part 1 — the parent row stays lit while its submenu is
+ * open. The primitive only styles :hover, so once the pointer crossed into
+ * the side card the parent row's fill cleared and the floating card read as
+ * unrelated. The Menu marks the open parent row with aria-expanded="true"
+ * (host-rendered, stable), so the same hover token re-applies for the
+ * submenu's whole lifetime. */
+body[data-dsh-sidebar-submenu] div[role="menu"] button[aria-expanded="true"] {
+  background: var(--dsw-alias-interactive-bg-hover);
+}
+
+/* Visual tether, part 2 — the side card hugs its parent (10px → 4px gap),
+ * with the ::before hover corridor narrowed to match. Ordered BEFORE the
+ * flip overrides below so their left/right flips still win. */
+body[data-dsh-sidebar-submenu] div[role="menu"] div[role="menu"] {
+  left: calc(100% + 4px);
+}
+
+body[data-dsh-sidebar-submenu] div[role="menu"] div[role="menu"]::before {
+  left: -4px;
+  width: 4px;
+}
+
 body[data-dsh-sidebar-submenu~="down"] div[role="menu"] div[role="menu"] {
   top: -4px;
   bottom: auto;
@@ -140,11 +163,11 @@ body[data-dsh-sidebar-submenu~="down"] div[role="menu"] div[role="menu"] {
 
 body[data-dsh-sidebar-submenu~="left"] div[role="menu"] div[role="menu"] {
   left: auto;
-  right: calc(100% + 10px);
+  right: calc(100% + 4px);
 }
 
 /* The hover bridge follows the card to the left side of the parent row. */
 body[data-dsh-sidebar-submenu~="left"] div[role="menu"] div[role="menu"]::before {
   left: auto;
-  right: -10px;
+  right: -4px;
 }