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-free.md

@@ -0,0 +1,25 @@
+---
+id: process-free
+name: Process.free
+category: process
+kind: namespace-method
+tokens: Process.free
+sig: Process.free(handle) -> void
+tip: Let a process handle go.
+order: 3
+ns: Process
+member: free
+---
+
+Releases the handle so it can be reused. It does <strong>not</strong> stop a child that is still running — use <a href="process-kill.html"><code>Process.kill</code></a> first for that; such a child keeps running and is reaped by the runtime when it ends, so it never lingers as a zombie. After <code>free</code>, polling the old handle answers <code>255</code>.
+
+```ludic
+program Demo {
+  entry {
+    let h = Process.spawn("/bin/sh", ["-c", "exit 0"])
+    var code = Process.poll(h)
+    while code == -1 { code = Process.poll(h) }
+    Process.free(h)
+  }
+}
+```