Function spawn_rebuild_worker 

pub fn spawn_rebuild_worker<F>(
    livereload: LiveReload,
    rebuild: F,
) -> RebuildWorker
where
    F: FnMut() -> Result<Vec<String>, RebuildError> + Send + 'static,

Spawn the rebuild worker on a dedicated OS thread and return the owned handle used to shut it down. The watcher closure should call tx.try_send(paths) with the changed paths for every filesystem event burst — failed sends (channel full) are intentional coalescing. Obtain tx via RebuildWorker::sender.

The closure runs synchronously on the worker thread, so it may use blocking I/O freely. It returns Ok(warnings) on success (a possibly-empty list of plain, color-free advisory lines to print under the rebuild line) or Err(RebuildError) on failure; the worker prints the error’s terminal rendering and broadcasts its plain message to connected browsers.

Keep the returned RebuildWorker alive until the server stops, then drop the watcher and call RebuildWorker::shutdown to wait for active writes.