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-save_to.md

@@ -0,0 +1,28 @@
+---
+id: http-save_to
+name: Http.save_to
+category: http
+kind: namespace-method
+tokens: Http.save_to
+sig: Http.save_to(handle, path) -> void
+tip: Stream the response body to a file.
+order: 16
+ns: Http
+member: save_to
+---
+
+Makes the response body of an <a href="http-open.html"><code>Http.open</code></a>ed request go straight to the file at <code>path</code> as it arrives, instead of into memory — the way to download something large (an update package of hundreds of megabytes) with a progress bar. Call it before <a href="http-send.html"><code>Http.send</code></a>; afterwards it does nothing.
+
+The file is written directly at <code>path</code>, with no temporary name: <code>Http.send</code> creates it (or truncates an existing one), and a failed or cancelled download leaves whatever had arrived. So download to a name of your own such as <code>"x.nupkg.part"</code> and rename it once <a href="http-poll.html"><code>Http.poll</code></a> reports a good status — a non-2xx reply's body (an error page) is written to the file too. If the file cannot be created the request finishes with status <code>0</code>, as does one whose write fails midway (a full disk).
+
+For such a request <a href="http-text.html"><code>Http.text</code></a> is empty (<code>null</code>) and <a href="http-body_len.html"><code>Http.body_len</code></a> is the number of bytes written. Follow progress with <a href="http-received.html"><code>Http.received</code></a> and <a href="http-expected.html"><code>Http.expected</code></a>. Freeing the request while it is pending cancels the download. The transfer asks for no compression (<code>Accept-Encoding: identity</code>) unless the request set that header, so the bytes counted are the bytes on disk. Files are limited to 2 GB.
+
+```ludic
+program Demo {
+  entry {
+    let h = Http.open("GET", "https://example.com/big.bin")
+    Http.save_to(h, "big.bin.part")
+    Http.send(h)
+  }
+}
+```