Title: MigrationBuilder core implementation for #519

What • Add MigrationBuilder for ZIO Schema migrations. • Add missing ADTs used by builder: MigrationAction, DynamicMigration. • Add DynamicValue.fromValue helper required by builder defaults.

Why • Fix missing types / helper methods referenced by MigrationBuilder causing CI compile failures.

How to verify • CI green: lint + tests on ubuntu-22.04 for Scala 11 / 25 passed. • Local: sbt schemaJVM/compile (optional)

Related • Fixes/Refs: #519 /claim #519

This PR implements the MigrationBuilder core for ZIO Schema migrations. Demo video: <(https://youtu.be/wDEtIrDohww)>