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-kill.md

@@ -0,0 +1,23 @@
+---
+id: process-kill
+name: Process.kill
+category: process
+kind: namespace-method
+tokens: Process.kill
+sig: Process.kill(handle) -> void
+tip: End a running child.
+order: 2
+ns: Process
+member: kill
+---
+
+Ends the child at once — <code>SIGKILL</code> on macOS, <code>TerminateProcess</code> on Windows — with no chance for it to save anything. A later <a href="process-poll.html"><code>Process.poll</code></a> answers <code>137</code> once it has gone. Does nothing for a child that has already ended, or a handle that is not one.
+
+```ludic
+program Demo {
+  entry {
+    let h = Process.spawn("/bin/sleep", ["30"])
+    Process.kill(h)
+  }
+}
+```