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/produced-files.ts

@@ -6,6 +6,7 @@
  * Kept dependency-free so the takeover logic is unit-testable and the
  * replica is easy to diff against upstream when it drifts.
  */
+import { isAbsolutePath } from './paths.ts'
 
 /** Paths a tool-result view reports as produced, by render intent. */
 export function producedPaths(view: unknown): readonly string[] {
@@ -76,10 +77,13 @@ export function selectProducedFiles(owner: unknown): readonly string[] | null {
   return paths.length === 0 ? null : paths
 }
 
-/** Resolve a (possibly relative) path against the session cwd for the sidebar. */
+/**
+ * Resolve a (possibly relative) path against the session cwd for the sidebar.
+ * Absolute detection mirrors the host (see client/paths.isAbsolutePath):
+ * POSIX roots, drive letters and UNC shares must not be joined onto the cwd.
+ */
 export function resolveSidebarPath(cwd: string | undefined, path: string): string {
-  const absolute = path.startsWith('/') || /^[A-Za-z]:[\\/]/.test(path)
-  if (absolute) return path
+  if (isAbsolutePath(path)) return path
   const base = cwd ?? ''
   if (base === '') return path
   const separator = base.includes('\\') ? '\\' : '/'