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/http/http-received.md

@@ -0,0 +1,25 @@
+---
+id: http-received
+name: Http.received
+category: http
+kind: namespace-method
+tokens: Http.received
+sig: Http.received(handle) -> int
+tip: Body bytes received so far.
+order: 17
+ns: Http
+member: received
+---
+
+Returns how many bytes of the response body have arrived. For a request given <a href="http-save_to.html"><code>Http.save_to</code></a> it is updated live while the request is pending — the bytes already written to the file — which is what a progress bar reads. For an ordinary in-memory request it is <code>0</code> until the reply lands, then the body length. <code>0</code> before <code>Http.send</code>.
+
+```ludic
+program Demo {
+  var dl: int = 0
+  handler Bar phase Render {
+    let got = Http.received(handle: dl)
+    let all = Http.expected(handle: dl)
+    if all > 0 { Screen.bar(x: 8, y: 8, width: 200, height: 6, value: got / 1024, max: all / 1024, color: 0x55cc77, back: 0x222222) }
+  }
+}
+```