Several tests inject a posix platform but resolve paths or executable names
through the host, so on Windows they see backslashes and tailscale.exe where
they assert forward slashes and tailscale.

- tailscale: pin HostProcessPlatform to linux in the spawner test layer.
- desktop DesktopEnvironment/DesktopAssets/DesktopAppIdentity and ssh config:
  provide NodePath.layerPosix alongside NodeServices so Path matches the
  posix-shaped fixtures.
- ClaudeSkills: hand skillOverrideSettingsPaths the Path implementation for
  the platform under test, with a real win32 fixture for the win32 case.
- ProjectFaviconResolver/ServerSecretStore: compare with path.join/basename
  instead of endsWith("/x").
- mobile-showcase: resolveAndroidSdkRoot joins with the path module for the
  platform argument it already takes.

Co-Authored-By: Claude Code <noreply@anthropic.com>