Expressing async generators as bidirectional streams

[dglazkov]

One neat thing that fell out of the adventures in google/breadboard-ai#132 is that there might be a way to plumb the run of a board down to invoked boards using this trick:

• express semantics of the async generator as a bidirectional stream (basically, a swizzled pair of ReadableStream and WritableStream that work in concert)
• teach run to think in terms of this bidirectional stream, rather than async generator. Obvs, they are identical semantically, so the consumer should be able to keep using async generator if they want to.
• plumb the bidirectional stream to NodeHandlerContext
• use that stream as the "run" in the invoked boards

This would allow the consumer of the top-level run see what's happening inside each board, proxy nodes, replace them, etc.

[seefeldb]

Not sure I follow.

Also, didn't you change proxy-ing nodes to something more implicit, i.e. nodes that themselves proxy using an ambient transport protocol, e.g. defined at the top-level (where that transport is known) as a runtime kit? Or is this what you meant?

FWIW, the other instructive scenario to consider is a server side process that needs to wait for a long time for some event (e.g. human approval step, new data coming in from somewhere, etc.) and should go to sleep in the meantime. Have that interrupt come from deep in the stack.