Function collect_referenced_files 

fn collect_referenced_files(
    md: &str,
    md_path: &Path,
    input_dir: &Path,
) -> Vec<(PathBuf, PathBuf)>

Parse md for local link and image targets that are not other Markdown files, resolve them relative to md_path’s parent directory, and return those that exist as files within input_dir.

Returns a list of (absolute_source_path, relative_path_from_input_dir) pairs. The caller uses the relative path to mirror the asset at the same position inside the output directory, keeping all relative URLs in the rendered HTML valid.

Skipped silently:

• Remote URLs (://), data URIs, and fragment-only refs (#…).
• .md files — those are rendered to .html, not copied.
• Refs that do not resolve to an existing file.
• Refs that resolve to a file outside input_dir (a warning is logged to the caller instead).