Feat: replace BlockCell with watch channel

[julio4]

📝 Summary

With #394, try_build is now an async method and we can use proper async channels and .await for flashblock timing signals.

• Replaced BlockCell with tokio::sync::watch channel. The builder publishes its best payload to the watch channel after each completed build (fallback block + each flashblock).
• resolve_kind can reads the latest best payload value instantly with borrow.
  • ResolvePayload resolves immediately with the pre-fetched value instead of acquiring lock/polling channel.
  • However if there's no value yet (fallback block not published), it waits on watch::Receiver::changed() until the first Some(best_payload) is published
• deprecate build_complete field (we never poll it anywhere).

💡 Motivation and Context

The current BlockCell implementation use Mutex<Option<T>> + Notify for sharing the best payload between the builder task and engine get payload. WaitForValue::poll calls cx.waker().wake_by_ref() on every poll when no value which i think produces a waker spin loop instead of yielding back to executor and Mutex adds unnecessary resource locks (as we have only one directional PayloadBuilder publisher -> PayloadResolve consumer)