Function append_safe_relative_components 

pub fn append_safe_relative_components(
    base: PathBuf,
    remainder: &Path,
) -> Result<PathBuf, RelativePathNormalizeError>

Append unresolved remainder components onto a resolved base.

Matches CLI canonicalize_existing_ancestor remainder handling:

• Normal → push
• CurDir → ignore
• ParentDir / Prefix / RootDir → refuse (escape / re-root)

Callers supply the already-canonical existing ancestor as base and the non-existing tail as remainder (from path.strip_prefix(ancestor)).