Proposal: Optional BoxLite sandbox backend (experimental)

## Summary

Could we add [BoxLite](https://github.com/boxlite-ai/boxlite) as an **optional** sandbox backend?

This is not a replacement request. Existing runtime defaults can stay as-is.

## Why

- Extra isolation option (micro-VM based).
- Good fit for projects that already have runtime abstraction.
- Opt-in only, so no disruption for existing users.

## Tiny example (Node.js)

```ts
import { SimpleBox } from '@boxlite-ai/boxlite';

async function run(i: number) {
const box = new SimpleBox({ image: 'alpine:latest', autoRemove: true });
try {
await box.exec('sh', '-lc', 'echo ok');
const m = await box.metrics();
console.log(`[box ${i}] totalCreateDurationMs=${m.totalCreateDurationMs}`);
} finally {
await box.stop();
}
}

async function main() {
await Promise.all([1, 2, 3].map(run));
}
main();
```

Warm startup is typically under 200ms.
Cold starts can be higher depending on image/cache state.

## Suggested scope

1. Add optional `boxlite` backend.
2. Keep current runtime behavior unchanged unless explicitly selected.
3. Validate parity for mounts, timeout, and cleanup lifecycle.