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

tests/paths.spec.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
-import { relativeTo } from '../src/client/paths.ts'
+import { isAbsolutePath, isWindowsStylePath, relativeTo } from '../src/client/paths.ts'
 import { resolveSidebarPath } from '../src/client/produced-files.ts'
+import { htmlUrl } from '../src/client/api.ts'
 
 describe('path helpers', () => {
   it('derives relative paths under the cwd (and "." for the cwd itself)', () => {
@@ -32,4 +33,57 @@ describe('path helpers', () => {
     expect(resolveSidebarPath('C:\\work\\proj', 'C:\\abs\\x.ts')).toBe('C:\\abs\\x.ts')
     expect(resolveSidebarPath('C:\\work\\proj\\', 'C:\\abs\\x.ts')).toBe('C:\\abs\\x.ts')
   })
+
+  it('keeps UNC produced paths absolute instead of joining them onto the cwd', () => {
+    // Pure client function: UNC detection is platform-independent, so these
+    // assertions run on every host without a platform guard.
+    expect(resolveSidebarPath('C:\\work\\proj', '\\\\server\\share\\abs\\x.ts'))
+      .toBe('\\\\server\\share\\abs\\x.ts')
+    expect(resolveSidebarPath('C:\\work\\proj', '//server/share/abs/x.ts'))
+      .toBe('//server/share/abs/x.ts')
+    // A relative path under a UNC cwd joins with backslashes.
+    expect(resolveSidebarPath('\\\\server\\share\\proj', 'src/a.ts'))
+      .toBe('\\\\server\\share\\proj\\src/a.ts')
+  })
+
+  it('mirrors the host absolute-path notion without node:path', () => {
+    expect(isAbsolutePath('/abs/x.ts')).toBe(true)
+    expect(isAbsolutePath('C:\\abs\\x.ts')).toBe(true)
+    expect(isAbsolutePath('C:/abs/x.ts')).toBe(true)
+    expect(isAbsolutePath('\\\\server\\share\\x.ts')).toBe(true)
+    expect(isAbsolutePath('//server/share/x.ts')).toBe(true)
+    expect(isAbsolutePath('C:relative.ts')).toBe(false)
+    expect(isAbsolutePath('rel/x.ts')).toBe(false)
+  })
+
+  it('detects windows-style session paths (platform signal for the html route)', () => {
+    expect(isWindowsStylePath('C:\\work\\proj')).toBe(true)
+    expect(isWindowsStylePath('C:/work/proj')).toBe(true)
+    expect(isWindowsStylePath('\\\\server\\share\\proj')).toBe(true)
+    expect(isWindowsStylePath('//server/share/proj')).toBe(false) // ambiguous form needs the drive/backslash signal
+    expect(isWindowsStylePath('/Users/me/code')).toBe(false)
+  })
+
+  it('htmlUrl marks UNC paths from the file path itself (cwd-independent signal)', () => {
+    // cwd not hydrated yet: the unambiguous backslash UNC path must still
+    // keep its '//' marker, or win32 would decode /server/... against the
+    // current drive and the isWithin fence would reject the preview (403).
+    expect(htmlUrl({ sessionId: 's' }, '\\\\server\\share\\proj\\a.html'))
+      .toBe('/sidebar/html/s//server/share/proj/a.html')
+    // A forward-slash-form cwd (still Windows) does not mask the backslash path.
+    expect(htmlUrl({ sessionId: 's', cwd: '//server/share/proj' }, '\\\\server\\share\\proj\\a.html'))
+      .toBe('/sidebar/html/s//server/share/proj/a.html')
+    // Both cwd and path in the forward-slash UNC form: the '//' path matches
+    // the '//' cwd, so it is UNC and keeps the marker (win32 round-trips).
+    expect(htmlUrl({ sessionId: 's', cwd: '//server/share/proj' }, '//server/share/proj/a.html'))
+      .toBe('/sidebar/html/s//server/share/proj/a.html')
+    // A Windows drive cwd marks forward-slash UNC paths.
+    expect(htmlUrl({ sessionId: 's', cwd: 'C:\\work' }, '//server/share/proj/a.html'))
+      .toBe('/sidebar/html/s//server/share/proj/a.html')
+    // A POSIX session keeps forward-slash paths marker-free.
+    expect(htmlUrl({ sessionId: 's', cwd: '/home/me' }, '//server/share/a.html'))
+      .toBe('/sidebar/html/s/server/share/a.html')
+    expect(htmlUrl({ sessionId: 's', cwd: '/home/me' }, '/home/me/index.html'))
+      .toBe('/sidebar/html/s/home/me/index.html')
+  })
 })