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-expected.md

@@ -0,0 +1,25 @@
+---
+id: http-expected
+name: Http.expected
+category: http
+kind: namespace-method
+tokens: Http.expected
+sig: Http.expected(handle) -> int
+tip: The body length the server announced, or -1.
+order: 18
+ns: Http
+member: expected
+---
+
+Returns the length of the response body as the server announced it (<code>Content-Length</code>), or <code>-1</code> while that is not known. For a request given <a href="http-save_to.html"><code>Http.save_to</code></a> it is known as soon as the headers arrive, and stays <code>-1</code> for a reply sent without a length (chunked) — draw an indeterminate bar then. For an ordinary in-memory request it is <code>-1</code> until the reply lands, then the body length.
+
+```ludic
+program Demo {
+  entry {
+    let h = Http.open("GET", "https://example.com/big.bin")
+    Http.save_to(h, "big.bin.part")
+    Http.send(h)
+    print(Http.expected(h))   # -1: nothing has arrived yet
+  }
+}
+```