AsyncProjection

Async iterable of deltas that is also awaitable for the final value.

Uses an asyncio.Event to notify consumers of state changes. Each waiter — the awaitable (__await__) and each async iterator cursor — shares the event and re-checks its own condition on wake. The event is cleared before a waiter awaits, so stale "something happened" signals don't cause spin loops.

This is single-loop only — producers and consumers must share an event loop. If cross-thread wake is ever required, revert to a list-of-futures pattern with call_soon_threadsafe.