Development-time source reloading for a running GPUI runtime.

watch/2 watches explicitly listed Elixir source files, recompiles a changed file, and asks the runtime to render its existing windows again. Window assigns remain authoritative, so ordinary render/1, handle_event/3, and handle_info/2 edits take effect without reopening the native window.

A watcher may receive a :notify PID. After every attempted file reload it receives {:gpui_reload, watcher, path, {:ok, modules}} or {:gpui_reload, watcher, path, {:error, reason}}. Files are parsed before compilation so syntax failures cannot unload or partially redefine the live root module.

This facility is intended for trusted local development source. It requires the optional :file_system dependency; applications that do not use source reload do not need to start or include it. The watcher does not reload native code, remount applications, or migrate changed state shapes.