Closes #519 /claim #519

Greeting @jdegoes,

Here’s my implementation of the purely algebraic Schema Migration System. I focused on building a reliable, serializable, and reversible system for handling structural schema transformations.

Overview of the Approach

1. Core Data Layer (Untyped & Serializable)

• I introduced MigrationError to provide clear error messages with proper path tracing.
• Implemented MigrationAction, which covers all 14 required path based transformations (using DynamicOptic). Each action also supports a structural reverse operation.
• Built DynamicMigration as the core engine that ties everything together. It follows important properties like identity, associativity, and reversibility.

2. Typed API for Users

• Created Migration[A, B] to safely handle transformations from one type to another (A → DynamicValue → B) using schemas.
• Added a MigrationBuilder to collect and apply actions. For now, it works directly with DynamicOptic.

Design Decisions

As mentioned in the issue, I’ve intentionally deferred macro-based selectors (like _.address.street). Supporting macros that work properly across both Scala 2.13 and Scala 3 is quite complex and deserves its own focused effort. Instead, I prioritized building a solid and fully serializable foundation first (DynamicMigration + DynamicOptic). This ensures we have a strong core system in place, and it will be easier to add macro based syntax on top later.

Testing & Validation

• Added around 70 test cases across:
  • MigrationActionSpec
  • DynamicMigrationSpec
  • MigrationSpec
• Verified key properties like identity, associativity, and reversibility
• Confirmed that the code works on both Scala 2.13 and Scala 3.7+