feat: 统一路径处理——UNC/软链接分类/html 路由平台守卫（#65 #67 #43 #79 #115）

- fs-tree: requireAbsolute 改用 path.isAbsolute（win32 UNC 放行、C:foo 仍拒）；
  目录列表仅对 symlink 行 stat 一次，暴露 isSymlink/broken，目录软链接可展开
- html-route: UNC 路径经 '//' 标记编码往返；标记仅在 Windows 风格会话启用，
  POSIX '//' 绝对路径不再被误判（平台守卫）
- client: isAbsolutePath/isWindowsStylePath 镜像 + resolveSidebarPath 识别 UNC，
  htmlUrl 以会话 cwd 形态传递平台信号；FsEntry 增加 isSymlink/broken
- explorer: 软链接行链接徽标、悬空链接标红 + tooltip（issue #65）
- 测试: tests/platform.ts 守卫助手；win32 专属用例改为 skipIf 可见跳过，
  补 POSIX 反向分支与 UNC 矩阵；新增 fs-tree-symlink.spec

src/client/api.ts

@@ -8,6 +8,7 @@
  */
 import { encodeHtmlUrl } from '../html-route.ts'
 import type { BrowserProbeResult } from './browser.ts'
+import { isWindowsStylePath } from './paths.ts'
 
 /** One wire failure. */
 export class SidebarApiError extends Error {
@@ -25,6 +26,10 @@ export interface FsEntry {
   path: string
   isDir: boolean
   hidden: boolean
+  /** Whether the row is a symlink; `isDir` then describes the link's target. */
+  isSymlink: boolean
+  /** For symlinks: the target is missing or unreadable (stat failed). */
+  broken: boolean
 }
 
 /** Git status entry (host git shape). */
@@ -207,9 +212,13 @@ function fileUrl(scope: SessionScope, path: string, download: boolean): string {
   return `/sidebar/file?${params.toString()}`
 }
 
-/** Absolute URL of the HTML preview route (see html-route.ts): the path is
- *  fully encoded so the previewed page's relative assets resolve back into
- *  the same route with the session scope intact. */
+/**
+ * Absolute URL of the HTML preview route (see html-route.ts): the path is
+ * fully encoded so the previewed page's relative assets resolve back into
+ * the same route with the session scope intact. The session cwd's shape
+ * supplies the platform guard for the UNC marker (the browser OS may differ
+ * from the host OS, so no client-side platform detection is used).
+ */
 export function htmlUrl(scope: SessionScope, path: string): string {
-  return encodeHtmlUrl(scope.sessionId, path)
+  return encodeHtmlUrl(scope.sessionId, path, isWindowsStylePath(scope.cwd ?? ''))
 }