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/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.