fix(quarto-core): handle Windows paths in !path metadata adjustment

adjust_paths_to_document_dir rebases !path metadata values (css,
resources, bibliography, ...) relative to the document directory. Two
Windows-only defects lived in that one function:

- The rebased value is stringified with to_string_lossy, so on Windows
  pathdiff's native separators leaked into the value. Since it is used
  verbatim in HTML hrefs (a css: !path <link>), the emitted attribute
  became href="..\..\shared\styles.css" instead of forward slashes.
  Normalize at the emission site with quarto_util::to_forward_slashes,
  the same helper the SCSS and JSON-writer separator fixes reuse.

- The relative-path guard used Path::is_relative, which on Windows is
  false only for drive-prefixed paths; a POSIX-absolute value like
  /usr/share/base.css is not is_absolute there, so it was wrongly
  rebased into ../../../usr/share/base.css. Guard on is_rooted
  (has_root) instead, matching the helper's documented purpose.

The suite's dir-metadata-paths fixture and the existing
directory_metadata_tests (previously red on Windows, green elsewhere)
now pass on all platforms.

Author: cderv

crates/quarto-core/src/project/mod.rs

@@ -230,14 +230,20 @@ fn adjust_paths_recursive(value: &mut ConfigValue, metadata_dir: &Path, document
     match &mut value.value {
         ConfigValueKind::Path(path_str) => {
             let path = PathBuf::from(&*path_str);
-            // Only adjust relative paths (not absolute, not URLs)
-            if path.is_relative()
+            // Only adjust relative paths (not absolute, not URLs). Use
+            // `is_rooted` (has_root), not `Path::is_relative`: on Windows a
+            // POSIX-absolute path like `/usr/share/base.css` is not
+            // `is_absolute` (no drive prefix) and would be wrongly rebased.
+            if !quarto_util::is_rooted(&path)
                 && !path_str.starts_with("http://")
                 && !path_str.starts_with("https://")
             {
                 let abs_path = metadata_dir.join(&path);
                 if let Some(adjusted) = pathdiff::diff_paths(&abs_path, document_dir) {
-                    *path_str = adjusted.to_string_lossy().into_owned();
+                    // The adjusted value is used verbatim in HTML hrefs (e.g. a
+                    // `css: !path` <link>), so it must use forward slashes on
+                    // every platform; pathdiff yields native separators.
+                    *path_str = quarto_util::to_forward_slashes(&adjusted);
                 }
             }
         }