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/paths.ts

@@ -3,8 +3,38 @@
  * the session cwd (for the @-reference button and "copy relative path").
  * The fs-tree joins with '/' even on Windows, so both separators normalize
  * to '/' before comparison.
+ *
+ * This module is dependency-free (no node:path in the client bundle): the
+ * host is the authority for path semantics, so these mirrors deliberately
+ * accept a SUPERSET of absolute forms — anything a Windows host would emit
+ * (drive letters, UNC) plus POSIX roots. A form the host would reject
+ * (e.g. a backslash UNC path on a POSIX host) passes through here and then
+ * fails loudly in the host's requireAbsolute instead of being silently
+ * joined onto the cwd.
  */
 
+/**
+ * Whether a path looks like it belongs to a Windows-style session: a drive
+ * letter or any backslash (UNC shares, and drive paths in general).
+ * Used as the platform signal for the html route's UNC marker — the client
+ * cannot read process.platform (the browser OS may differ from the host),
+ * but the session cwd shape is authoritative.
+ */
+export function isWindowsStylePath(path: string): boolean {
+  return path.includes('\\') || /^[A-Za-z]:[\\/]/.test(path)
+}
+
+/**
+ * Mirror of the host's absolute-path notion (see fs-tree.requireAbsolute):
+ * POSIX roots, Windows drive letters, and Windows UNC network shares in
+ * both backslash (`\\server\share\...`) and forward-slash
+ * (`//server/share/...`) form. Deliberately a superset — see the module
+ * comment — so a produced UNC path is never joined onto the cwd.
+ */
+export function isAbsolutePath(path: string): boolean {
+  return path.startsWith('/') || /^[A-Za-z]:[\\/]/.test(path) || /^[\\/]{2}[^\\/]/.test(path)
+}
+
 /**
  * The path relative to the session's working directory.
  * @param cwd - the explorer root (absolute).