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/process/process-poll.md

@@ -0,0 +1,32 @@
+---
+id: process-poll
+name: Process.poll
+category: process
+kind: namespace-method
+tokens: Process.poll
+sig: Process.poll(handle) -> int
+tip: -1 while running, else the exit code.
+order: 1
+ns: Process
+member: poll
+---
+
+Returns <code>-1</code> while the child is still running and its exit code once it has ended; it never waits. The code is kept, so asking again gives the same answer until <a href="process-free.html"><code>Process.free</code></a>.
+
+A child ended by a signal (macOS) answers <code>128</code> plus the signal number, as a shell reports it — <code>137</code> after <a href="process-kill.html"><code>Process.kill</code></a>, on both platforms. On Windows the code is the child's DWORD exit code as an <code>int</code>, so a crash such as <code>0xC0000005</code> arrives as a negative number (test for <code>!= 0</code>, not <code>&gt; 0</code>); a code of <code>0xFFFFFFFF</code> is reported as <code>255</code> so that <code>-1</code> always means "still running". A handle that is not one answers <code>255</code>.
+
+```ludic
+program Demo {
+  var game: int = -1
+  handler Watch phase Update {
+    if game >= 0 {
+      let code = Process.poll(handle: game)
+      if code >= 0 or code < -1 {
+        if code != 0 { print("the game crashed") }
+        Process.free(handle: game)
+        game = -1
+      }
+    }
+  }
+}
+```