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>

docs/language/app/_section.md

@@ -4,4 +4,4 @@ title: App
 order: 48
 ---
 
-The running application, as distinct from its window. Today that is the boot splash: the runtime raises it before <code>main</code> from the game's asset pack, and <a href="app-splash_hide"><code>App.splash_hide</code></a> takes it down when the game has something to show instead.
+The running application, as distinct from its window. Today that is the boot splash: the runtime raises it before <code>main</code> from the game's asset pack, and <a href="app-splash_hide"><code>App.splash_hide</code></a> takes it down when the game has something to show instead. <a href="app-window_hide"><code>App.window_hide</code></a> and <a href="app-window_show"><code>App.window_show</code></a> take the game's own window off the screen and bring it back without closing it — a launcher stepping aside while the game it started runs.

docs/language/app/app-window_hide.md

@@ -0,0 +1,35 @@
+---
+id: app-window_hide
+name: App.window_hide
+category: app
+kind: namespace-method
+tokens: App.window_hide
+sig: App.window_hide() -> void
+tip: Take the game's window off the screen.
+order: 3
+ns: App
+member: window_hide
+---
+
+Takes the game's window off the screen without closing it: its GL context or Vulkan surface, its size and its contents are kept, and the program goes on running — a hidden window does not end the run. <a href="app-window_show.html"><code>App.window_show</code></a> brings it back. It is how a launcher steps aside while the game it started with <a href="../process/process-spawn.html"><code>Process.spawn</code></a> runs.
+
+Keep the frame loop going while hidden so the child can be polled, but there is nothing to see: skip the drawing, and throttle the loop yourself — a hidden window is not paced by the display, so an unthrottled loop spins. It does not touch the boot splash (<a href="app-splash_hide.html"><code>App.splash_hide</code></a> does). Safe to call when there is no window yet, and twice. Headless there is no window and the call lowers to nothing.
+
+```ludic skip
+program Launcher {
+  var game: int = -1
+  function play() -> void {
+    game = Process.spawn(game_path(), new []string)
+    if game >= 0 { App.window_hide() }
+  }
+  handler Watch phase Update {
+    if game >= 0 and Process.poll(game) != -1 {
+      Process.free(game)
+      game = -1
+      App.window_show()
+    }
+  }
+}
+```
+
+<code>orderOut:</code> on macOS; <code>ShowWindow(SW_HIDE)</code> on Windows, which also forgets any key held as the window went.

docs/language/app/app-window_show.md

@@ -0,0 +1,26 @@
+---
+id: app-window_show
+name: App.window_show
+category: app
+kind: namespace-method
+tokens: App.window_show
+sig: App.window_show() -> void
+tip: Bring the game's window back.
+order: 4
+ns: App
+member: window_show
+---
+
+Puts a window hidden by <a href="app-window_hide.html"><code>App.window_hide</code></a> back on screen, in front and focused, as it was. Safe to call on a window that is already showing, when there is no window, and headless, where it lowers to nothing.
+
+```ludic skip
+program Demo {
+  entry {
+    App.window_hide()
+    wait_for_the_game()
+    App.window_show()
+  }
+}
+```
+
+On macOS <code>makeKeyAndOrderFront:</code> and the application is activated. On Windows <code>ShowWindow(SW_SHOW)</code> and <code>SetForegroundWindow</code>; Windows may refuse the focus to a program that is not in the foreground, in which case the window comes back and its taskbar button flashes instead.