Why: U4 ships the managed migration entry; the runbook is the operator-facing contract for inventory/backup/migrate, migration identity and resume rules, verification semantics, and the authorized-window switch sequence.
Evidence: backend branch feature/collab-storage-u4-migrator - integration tests green against real PostgreSQL (idempotent rerun, resume, foreign-data refusal, wrong-snapshot refusal, unknown-owner block).
Impact: docs only.
Refs: proposals/2026-10-11-collaboration-storage-and-distribution.md U4 and section 6.
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