Uploaded files can no longer open as an empty page

Uploading a file into a content collection and opening it right away could — very rarely — show an empty page instead of the file's content, and the page stayed empty until the collection was reloaded. The cause was a race between the upload's own read of the file and the file-system watcher's read of the moment the file was created: the stale, still-empty snapshot could be applied after the complete one and stick. The ordering guard is now applied atomically with the update itself, so the complete content always wins and the freshly uploaded file renders immediately.