Add migration dry-run output that identifies lossy or ambiguous state conversions

[Jagadeeshftw]

Problem

Migration failures are hard to review when operators cannot see which records would be lossy or ambiguous before execution.

Scope

Add a read-only migration dry run that reports counts, incompatible records, and required manual decisions without mutating storage.

Acceptance criteria

• Dry runs are deterministic and side-effect free.
• Every rejected conversion includes a stable reason code.
• The execution path refuses to proceed when a lossy record is detected.

Validation

Test clean, partially compatible, and irreconcilable datasets.