• Implemented the core schema migration system: a pure, serializable MigrationAction ADT (14 action cases) with DynamicOptic-based path targeting for arbitrarily nested structures
• Added DynamicMigration (untyped, serializable core) and Migration[A, B] (typed wrapper) with apply, composition (++), and structural reverse
• Provided MigrationBuilder DSL with macro-validated selector syntax (_.field.nested) for Scala 2.13 and Scala 3, accessed via Migration.newBuilder[A, B]
• Support for nested migrations via TransformCase, which applies a sub-list of actions relative to a variant case value
• Introduced MigrationError with DynamicOptic path information for diagnostics

/claim #519 closes #519