Why: The first trial reduced final CI churn, while later Dev merges exposed gaps in shared window records that must be addressed before broadening manual coordination.
Evidence: SELF Gitea API snapshot for PR #539–#548, current #518 comments, protection and open PR states; fixed first-trial report and current project rules reviewed.
Impact: Add a stage-4 design and evidence snapshot only; no queue activation, platform protection change, production task takeover, or workflow edit.
Refs: Dev issue #518; Documents 2026-10-14 merge queue trial report