`#spawn` decorator (fire-and-forget)

Fire-and-forget execution:

```lea
let logEvent = (event) ->
  sendToAnalytics(event)
#spawn

-- Returns immediately, doesn't block
logEvent({ type: \"click\" })
```

Needs to decide: error handling for unhandled rejections, interaction with structured concurrency (issue #126), and whether spawned functions should still be awaitable via a returned handle.

Migrated from `TODO.md`.