Summary

Implements the Schema Migration System for ZIO Schema 2 as specified in issue #519.

Changes Made

• Added DynamicMigration - pure, serializable migration representation
• Added MigrationAction ADT with all specified operations:
  • Record operations: AddField, DropField, Rename, TransformValue, Mandate, Optionalize, ChangeType, Join, Split
  • Enum operations: RenameCase, TransformCase
  • Collection operations: TransformElements, TransformKeys, TransformValues
• Added Migration[A, B] - typed user-facing API
• Added MigrationBuilder - fluent builder for constructing migrations
• Added DynamicMigrationInterpreter - functional interpreter for migrations
• Added MigrationError - path-based error reporting
• Added PrimitiveConversions - safe type coercions
• Added MigrationSpec - tests for basic functionality
• Added documentation in docs/reference/migration.md

Key Features

• Pure data: All migrations are represented as pure data, enabling serialization
• Reversible: Full support for structural reverse operations
• Composable: Migrations can be composed sequentially using ++
• Error reporting: Errors include path information for debugging
• Type-safe: Typed Migration[A, B] API

Implementation Notes

This is an initial implementation with:

• Core migration actions implemented
• Basic interpreter functionality
• Path-based transformations via DynamicOptic

Full macro support for selector extraction and complete serialization will be added in subsequent PRs.

/claim #519