[v2] Define the v1-to-v2 integration and release plan #2526

Summary

Produce the approved procedure for maintaining, testing, and merging v2 changes without disrupting remaining v1 releases or existing main-based automation.

Current working position

• Breaking v2 pull requests remain open against main while this procedure is unresolved.
• Pull request owners may rebase independently as main moves.
• No breaking v2 pull request merges until this issue records an approved plan.

Plan requirements

• Define how the open v2 pull requests are tested as a combined change set.
• Define the merge order and handling of conflicts between overlapping pull requests.
• Choose whether integration happens directly over a short merge window or through a temporary integration branch.
• Identify the final planned v1 release and whether any v1 servicing is required afterward.
• Preserve existing main-based release automation unless an explicit automation change is approved.
• Define v2 preview versioning, publishing, and validation.
• Define the criteria and responsible maintainers for promoting v2 to stable.
• Include freeze points, rollback handling, and the explicit gate allowing the first breaking change into main.

Completion

Maintainers approve and document a step-by-step procedure that can be followed when the implementation pull requests are ready. This issue is complete when the plan is actionable, not merely when alternatives have been discussed.