FLOWIP-071f: Sink receipts and receipt-gated progress

[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

• Sinks no longer report success for buffered work. A new Buffered delivery result holds contract state open until flush confirms the batch.
• Progress tracks two watermarks separately: how far the sink has read vs. how far durable receipt evidence extends. Completion and contracts use the receipt watermark only.
• Contract verification runs after flush, not before. The previous ordering let the final check pass before the last batch was committed.
• DeliveryContract clears all parent IDs on a receipt, not just the first, so future multi-parent receipts work without further changes.
• CsvSink uses the new buffering protocol: per-event Buffered receipts on consume, per-event Success receipts on flush.