[juhop-code]

Summary

Keep worker output metadata consistent with the frozen artifact when background aggregation reduces parts and rows. Both rewrite and compaction now stop merge selection and drain in-flight merges before measurement and FREEZE.

Changes

• Fail safely if a stable snapshot cannot be established; restore merges on success, failure and cancellation.
• Use a separate bounded drain deadline, independent of the ordinary merge optimization budget.
• Preserve all E2E log assertions by reading the Docker log-driver stream until worker exit.

Testing

• Formatting, go mod tidy, go vet ./..., go test ./... and rewrite race tests pass.
• Full E2E passed with ClickHouse 26.6 and 26.9-posthog, matching the CI matrix.
• Regression coverage includes aggregate row reduction, draining, timeout/error cleanup and cancellation.