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>

changes/launcher-runtime.md

@@ -0,0 +1,22 @@
+bump: minor
+type: feat
+**A launcher's runtime** — start a program, step aside while it runs, and download large
+files with a progress bar.
+
+- **`Process.*`** — `Process.spawn(path, args)` starts a program directly (no shell) with this
+  process's environment and working directory and returns at once; `Process.poll(h)` is `-1`
+  while it runs, then its exit code (`128 + signal` for a signal death, `137` after
+  `Process.kill`); `Process.free` lets a handle go. `posix_spawn` on macOS
+  (`runtime/native/process.ll`); `CreateProcessW` on Windows (`process_win.ll`), with each
+  argument quoted by the MSVC rules and no console window. Linked only into a program that
+  uses `Process.*`.
+- **`Http.save_to(h, path)`** — before `Http.send`, streams the response body straight into a
+  file (written at `path` itself, created on send) instead of memory; `Http.text` of such a
+  request is empty and `Http.body_len` is the bytes written. **`Http.received(h)`** and
+  **`Http.expected(h)`** (the `Content-Length`, or `-1`) are live while it is pending. macOS
+  streams through an `NSURLSession` with a delegate built at run time, Windows through the
+  WinHTTP read loop. Freeing a pending request now cancels it and parks its slot until the
+  worker has let go, rather than freeing what the worker is still writing.
+- **`App.window_hide()` / `App.window_show()`** — take the game's window off the screen and
+  bring it back without closing it or its GL / Vulkan surface; the run goes on while it is
+  hidden. No-ops headless and before there is a window.