Result migration support and manifest overlays

[hermabr]

Summary

• Added result-level migration primitives (ResultAdded, ResultRenamed, ResultRewrite) and exported them from the package root.
• Extended migration validation and result loading so result= migrations can rewrite cached result manifests without mutating the source bundle.
• Introduced result-manifest overlays and updated loading paths to prefer migrated overlays when present.
• Added coverage for result migration behavior, chained overlays, and invalid breaking/result combinations.

Testing

• Not run.
• Concrete checks added in tests: migrate an old result into a new schema, verify the source bundle manifest stays unchanged, and confirm the overlay is written.
• Concrete checks added in tests: ensure result migrations compose across multiple schema hops and that embedded child result migrations only affect the child's cached result.
• Concrete checks added in tests: verify class creation rejects breaking=True combined with result= migrations.