feat: add lower-level parallel streaming support

[Haricharanpanjwani]

Closes #534.

Summary

• make TaskBasedParallelAction stream-capable by extending it through Burr's existing streaming container flow
• add a StreamItem event type plus sync/async subgraph streaming helpers on SubGraphTask
• preserve stable task ordering for reduce(...) while allowing intermediate stream events to arrive as tasks complete
• add toggles for intermediate_stream_outputs and intermediate_nodes to control how much subgraph output is surfaced
• document the new lower-level parallel streaming capability in the parallelism guide

Testing

• python3 -m py_compile burr/core/parallelism.py tests/core/test_parallelism.py
• XDG_CACHE_HOME=/tmp/xdg-cache UV_CACHE_DIR=/tmp/uv-cache uv run --with pytest --with pytest-asyncio --with pydantic pytest tests/core/test_parallelism.py -q -k "streaming"
• XDG_CACHE_HOME=/tmp/xdg-cache UV_CACHE_DIR=/tmp/uv-cache uv run --with pytest --with pytest-asyncio --with pydantic pytest tests/core/test_parallelism.py -q

Notes

• This scopes the fix to the lower-level TaskBasedParallelAction capability described in the issue so the existing Application.stream_result(...) and Application.astream_result(...) APIs can surface parallel subgraph events without introducing a separate public streaming entrypoint.
• The broader queue/executor abstraction ideas from the issue body can still build on top of this in follow-up work if we want specialized multiprocessing or distributed backends later.