fix: symlink 探测有界并发 + UNC 标记平台中性化（回应 CR/自审）

- fs-tree: symlink 目标 stat 从串行改为 32 并发工作池（readdir 流关闭后统一
  探测），symlink 密集/网络目标目录不再累加延迟；顺序与分类语义不变
- html-route: 解码不再重建反斜杠 UNC，统一重建为平台中性的 //server/share/...
  前斜杠形态（win32/POSIX 均正确 resolve），从而删除 windowsPathStyle 平台信号
  与 client isWindowsStylePath 助手；同时解决无 cwd 时正斜杠 UNC 的往返盲点
- 测试/文档同步；移除未使用的 isPosix

src/client/api.ts

@@ -8,7 +8,6 @@
  */
 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 {
@@ -212,29 +211,14 @@ function fileUrl(scope: SessionScope, path: string, download: boolean): string {
   return `/sidebar/file?${params.toString()}`
 }
 
-/** A forward-slash UNC share (`//server/share/...`) — ambiguous with a POSIX
- *  absolute path, so it only counts as Windows-style when the session cwd is
- *  itself a forward-slash UNC (see htmlUrl). */
-function isForwardSlashUnc(path: string): boolean {
-  return /^\/\/[^/]/.test(path)
-}
-
 /**
  * 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 platform guard for the
- * UNC marker comes from the session cwd's shape (the browser OS may differ
- * from the host OS, so no client-side platform detection is used), extended
- * by the file path itself: an unambiguous backslash UNC path keeps its '//'
- * marker even while the cwd is still hydrating or arrives in forward-slash
- * form. A forward-slash `//` path is only marked when the cwd is itself a
- * forward-slash UNC — a POSIX session keeps `//server/share/...` as an
- * ordinary absolute path.
+ * the same route with the session scope intact. The UNC marker is
+ * platform-neutral — the host's requireAbsolute resolves the decoded
+ * forward-slash `//server/share/...` form on both win32 and POSIX — so no
+ * client-side platform signal is needed.
  */
 export function htmlUrl(scope: SessionScope, path: string): string {
-  const cwd = scope.cwd ?? ''
-  const windowsPathStyle = isWindowsStylePath(cwd)
-    || isWindowsStylePath(path)
-    || (isForwardSlashUnc(path) && isForwardSlashUnc(cwd))
-  return encodeHtmlUrl(scope.sessionId, path, windowsPathStyle)
+  return encodeHtmlUrl(scope.sessionId, path)
 }

src/client/paths.ts

@@ -5,25 +5,14 @@
  * 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
+ * host is the authority for path semantics, so this mirror deliberately
+ * accepts 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