awaitInitialNodeDb

Suspend until the device finishes streaming its NodeDB during handshake, then return the current node-snapshot map.

This waits for RadioClient.connection to reach ConnectionState.Connected (which the engine only signals after the handshake completes) and then snapshots the engine's NodeDB.

Cancellation is cooperative: cancelling the caller's coroutine simply unsubscribes from the flow; the engine is unaffected.