FLOWIP-071h: Fan-in ancestry propagation at merge boundaries

[rocketpages]

Checklist

• I read CONTRIBUTING.md
• All commits are signed off (DCO): git commit -s
• I ran: cargo fmt --all
• I ran: cargo test --workspace

Summary

• Join outputs now record where they came from on both sides, not just the stream side
• Drain-time outputs preserve ancestry even when the join produced zero outputs beforehand
• Error journal entries carry the same combined lineage as normal outputs
• All reference inputs are observed as they arrive so nothing is silently dropped from the record