feat: add suspendTerminal() to hand the terminal to a child process

[costajohnt]

Fixes #956.

Adds suspendTerminal() on useApp() so an Ink app can temporarily hand the terminal to a child process (e.g. $EDITOR, less, fzf) and then restore its own terminal state with a full redraw. This is the scoped terminal handoff discussed in the issue, rather than a stdout-only suspend.

API

Callback form (Ink restores the terminal even if the callback throws):

const {suspendTerminal} = useApp();

await suspendTerminal(async () => {
	await runEditor();
});

Disposable form, including await using:

await using suspension = await suspendTerminal();
await runEditor();

Behavior

On suspend, Ink flushes pending output, stops consuming input, and restores the terminal modes the child expects (raw mode off, cursor visible, bracketed paste off, alternate screen exited, kitty keyboard protocol off). On resume it reapplies its own terminal state and forces a full redraw rather than diffing against the stale pre-suspension frame. resume() is async because restoring ownership may need to wait for ordered writes and a redraw. Suspending while already suspended throws, and in non-interactive output the callback still runs but no terminal handoff happens.

Tests and docs

test/suspend-terminal.tsx covers the callback, disposable, and await using forms, throw-restores, the already-suspended error, alternate-screen exit and re-enter, the begin-suspend rollback path, keeping Ink off the terminal while suspended, and a PTY integration test that hands the terminal to a real child process and asserts the redraw on resume. There is a runnable example under examples/suspend-terminal/, and the README documents the API under useApp().