MM-31: Add legacy memory dry-run migration, export, and rollback

Parent

#771

Checkpoint

MC-9 Core release candidate

What to build

Provide dry-run migration and reports for legacy memory/context state, mixed-version operation, complete export/delete, upgrade/downgrade, and rollback.

Acceptance criteria

• Dry-run is side-effect free and reports every planned change.
• Migration is idempotent and crash resumable.
• Old/new mixed state remains readable.
• Export/delete cover profile, proposals, episodes, manifests, archive refs, working state, and caches.
• Rollback drill restores a working prior version without data loss.

Required tests

• Dry-run/idempotency
• mixed state
• crash/restart
• upgrade/downgrade
• export/delete completeness

Benchmark

Migration performance and post-migration lifecycle/continuity smoke; no quality comparison needed.

Blocked by

• [MM-25] Add MemoryPropose host gate and full profile lifecycle #805 (MM-25)
• [MM-27] Harden source-bearing compact boundaries for resume and fork #807 (MM-27)

Rollout and rollback

Versioned backup and explicit rollback command are mandatory before migration apply.