project_dir rebuilt the folder with `/` and dropped a UNC share's leading
`\`, so a project opened from \server\share\Job looked for its moved
footage under the relative path server/share/Job and never found it. It now
keeps the path's own separator and its leading `/` or `\`.
The relink tests built mixed-separator paths (`…\Job/p.ecproj`) and so failed
on Windows; they now use native paths and check the separator is kept.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>