Design alpha archive migration and compatibility tooling

[devkyato]

Goal

Provide a safe path for archives created by experimental alpha format revisions.

Acceptance criteria

• Document the compatibility policy for major/minor format changes.
• Add a read-only compatibility probe that never executes archive content.
• Design a migration command that verifies source and migrated archives fully.
• Preserve unknown optional metadata where practical.
• Add golden archives and corruption tests for every supported format revision.
• Never overwrite the source archive by default.

[devkyato]

v0.2.0 separates binary format 1.1 from manifest schema 2.0 and documents the intentional alpha compatibility break. Migration/compatibility tooling is not included yet, so this issue stays open rather than implying 0.1 archives have an automated path.