Document M2 atomic reindex rollout

[BigSimmo]

Summary

• Adds the dedicated M2 atomic reindex migration note before implementation.
• Documents API/config touchpoints, compatibility assumptions, migration shape, tests, and acceptance criteria.
• Keeps M2 separate from the 80/20 remediation PR.

Verification

• Docs-only change; no runtime checks were required.
• Source branch is based on origin/main and contains no M2 code changes.

Next Step

• Review the migration assumptions, then implement M2 in this branch with targeted reindex/retrieval tests.

[Copilot]

Pull request overview

Adds a planning/implementation note documenting the intended “M2 atomic reindex” rollout so that reindexing can complete atomically (preventing search/retrieval/telemetry from observing partially swapped index generations) before any code is changed.

Changes:

• Introduces a dedicated migration note describing current risks, touchpoints, and compatibility assumptions.
• Outlines a proposed “staged generation → single atomic cutover → cleanup” migration shape.
• Defines required tests and acceptance criteria to validate the rollout.

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