Function watch_directory 

pub async fn watch_directory(
    root: &Path,
    output_dir: &Path,
) -> Result<(), WatchError>

Watch root for file changes and trigger rebuilds into output_dir.

This is an async loop that runs until cancelled. On each batch of debounced file changes, it logs the changed paths and invokes an incremental rebuild (only changed files have their cache invalidated).

Arguments

• root - Directory to watch recursively.
• output_dir - Where to write rebuild output.