Document Core Data to SwiftData migration plan

[zigdanis]

What changed

• adds a documentation-only plan for replacing Core Data with SwiftData
• inventories the current shared App Group persistence behavior
• proposes a side-by-side, resumable, verified data migration
• defines implementation phases, validation, rollout, rollback, and cleanup criteria
• outlines the expected benefits, costs, and unresolved decisions

Why

The app and widget share an existing Core Data store containing user data and image references. A future SwiftData implementation needs an agreed approach that preserves those values, handles cross-process access, and keeps rollback safe.

Impact

No production code, schema, build settings, or runtime behavior changes in this PR. This is a planning artifact for a separately reviewed future implementation.

Validation

• reviewed the plan against the current Core Data model, gateway, stack, app/widget composition roots, and tests
• ran git diff --check

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