The front's broker path buffered the entire SSE response and sent one envelope, so
broker clients (lisa, and any OpenAI client via AISBF) got the whole reply at once.
Now the front streams it:

- streaming.py: stream_chunk_envelope uses event="chunk" with payload.chunk (the
  shape the AISBF relay consumes), finalize_stream uses event="done".
- dispatcher.py: extracted resolve_broker_request() (op routing + body decode +
  validation) shared by the buffered dispatch and the new streaming path, so they
  can't diverge; added BrokerDispatchError.
- app.py: broker_execute_stream() proxies to the engine with stream=True and yields
  each SSE chunk (sharing the per-model queue + in-flight tracking); start_broker
  wires client.stream_dispatcher.
- client.py: handle_message, for stream=true requests, relays each yielded chunk as
  a `chunk` envelope and ends with a `done` (instead of the single buffered reply).

Requires the matching AISBF change (send_request returns early for streaming and
the relay drains the chunk queue) — committed in the aisbf repo.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m