Plan Data Migrations & Schema Evolution

Background: Native [platform] app already in production using [SwiftData
/ Core Data / Room]. Current schema: [describe]. The change I need to
make: [add field / change type / split or merge entities / new
relationship].

Use case: Plan a safe schema migration that won't crash or lose data for
users updating from the old version.

Implementation details:
- iOS: lightweight vs heavyweight Core Data migration, or SwiftData
  schema versioning + migration plan—decide which my change requires
- Android: Room migration (auto-migration vs manual Migration), and what
  the change forces
- Preserving existing user data through the change
- What happens for users who skip versions (v1 → v3 directly)
- A rollback/safety story if the migration fails on a device
- How to test the migration against a realistic old database

Deliverable: The migration approach for my specific change, the migration
code/plan, the skip-version handling, the failure-safety strategy, and a
test plan that exercises the migration from the prior schema.