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-spawn.md

@@ -0,0 +1,27 @@
+---
+id: process-spawn
+name: Process.spawn
+category: process
+kind: namespace-method
+tokens: Process.spawn
+sig: Process.spawn(path, args) -> int
+tip: Start a program; never blocks.
+order: 0
+ns: Process
+member: spawn
+---
+
+Starts the program at <code>path</code> with the arguments in <code>args</code> (a <code>[]string</code>; the child's <code>argv[0]</code> is <code>path</code>) and returns a handle <code>&gt;= 0</code> straight away, or <code>-1</code> when the program could not be started — a missing file, a file that is not executable, or all 16 handles in use.
+
+No shell is involved, so every argument reaches the child exactly as written: spaces, quotes and <code>$</code> are not interpreted. The child inherits this process's environment and working directory. On Windows the arguments are joined into one command line quoted so the child's C runtime splits it back into the same list, the path and arguments cross as UTF-8 and are converted to UTF-16, and the child is started without a console window (a GUI program is unaffected; a console program runs hidden). Use an absolute path: there is no <code>PATH</code> search.
+
+```ludic
+program Demo {
+  entry {
+    let h = Process.spawn("/bin/sh", ["-c", "exit 3"])
+    print(h >= 0)
+  }
+}
+```
+
+See <a href="process-poll.html"><code>Process.poll</code></a> for the exit code.