stream: cache minimum cursor count in broadcast

Track how many broadcast consumers share the cached minimum cursor,
matching the share implementation. This lets buffer trimming avoid
calling getMinCursor() until the last minimum-cursor consumer
advances or detaches. Add coverage for fan-out trimming behavior.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #63322
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>

Author: trivikr