Function spawn_watcher 

pub fn spawn_watcher<F>(cfg: WatchConfig, on_event: F) -> Result<WatcherHandle>
where
    F: Fn(Vec<PathBuf>) + Send + 'static,

Start a debounced recursive watcher.

The closure on_event is invoked once per debounce window with the owned, deduplicated list of paths that changed outside any cfg.ignore root. If every event in a window targets an ignored subtree, the callback is not invoked.

Non-existent paths in cfg.paths are silently skipped; this matches the dev-server use case where some watched directories (e.g. an optional public/) may not exist yet.

Errors

Returns an error if the watcher cannot be created or if a path that exists cannot be watched (typically a permissions issue).