Implements the Schema Migration System for ZIO Schema 2, enabling pure, algebraic schema evolution where migrations are first-class, serializable data with compile-time validation.

• DynamicMigration ADT with MigrationAction operations (AddField, DropField, Rename, Join, Split, Transform)
• MigrationBuilder with SchemaExpr-first API and selector macros for type-safe path extraction
• TrackedMigrationBuilder with ValidationProof for true compile-time validation via .build
• DynamicMigrationInterpreter for applying migrations to DynamicValue
• Full reversibility with Resolved.inverse for automatic migration reversal
• PrimitiveConversions with 67 tested safe type coercions
• Nested migration support via DynamicOptic paths
• StructuralMigrationSpec - 36 new structural type tests
• Complete serialization: Schema[DynamicMigration] and Schema[MigrationAction]
• 1612+ tests covering all operations, laws, and edge cases

Fixes #519 /claim #519