[Kafka Streams runner] Use a single Impulse bootstrap topic with fan-out

## Summary

Currently every Impulse transform gets its own bootstrap topic (`__beam_impulse__`), because Kafka Streams rejects registering one topic on two source nodes and a pipeline can contain several Impulses (e.g. an empty `Create` plus the dummy branch `PAssert` adds).

Per review of #39380 this can be optimized: keep a single per-application bootstrap topic with one source node, and fan the created element out to all impulse processors from there, instead of one topic per transform.

Part of #18479.