Hugo: Symlink confinement bypass in os.ReadFile

Summary

**Affected versions:** v0.123.0 through v0.163.0. Earlier versions are not affected. **Fixed in:** v0.163.1. **Severity:** Medium. Requires the attacker to be able to place (or convince a site author to place) a symlink inside a mounted directory — for example, inside a locally-vendored theme under `themes/`. Themes mounted as Go modules from GitHub have symlinks stripped on download and are not affected. Multi-directory walks (e.g. content/asset walking) were not affected either; only direct lookups via `resources.Get` followed symlinks. **Description.** Hugo's virtual filesystem is designed so that files under a mount cannot reach outside the mount tree. A regression introduced in v0.123.0 caused `RootMappingFs.statRoot` to call `Stat` (which follows symlinks) instead of `Lstat`, so a direct `os.ReadFile "somefile"` where `somefile` was a symlink pointing outside the mount would return the target's contents. This effectively let a symlink planted inside a theme or local mount read arbitrary files reachable to the user running `hugo`.