feat(launcher): Process.*, Http.save_to/received/expected, App.window_hide/show

Process.spawn/poll/kill/free - non-blocking child processes with no shell: posix_spawn on
macOS (process.ll), CreateProcessW with MSVC-quoted arguments and no console window on
Windows (process_win.ll), linked only when a program uses Process.*.

Http.save_to streams a response body into a file (NSURLSession with a run-time delegate
class on macOS, the WinHTTP read loop on Windows); Http.received / Http.expected report
progress while it is pending. Freeing a pending request cancels it and parks the slot
until the worker has finished.

App.window_hide / App.window_show take the game's window off the screen and back without
closing it; the run goes on while hidden. Docs, examples, tests and a changeset.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

selfhost/backend/stdlib/emit_pak.ludic

@@ -120,6 +120,8 @@ function emit_pak_prelude() -> void {
     emith("declare void @splash_show(ptr, i32, i32)\n")
     emith("declare void @splash_hide()\n")
     emith("declare void @app_set_icon(ptr, i32)\n")
+    emith("declare void @app_window_hide()\n")
+    emith("declare void @app_window_show()\n")
   }
 
   emit_pak_mount()