Implements #519 /claim #519

Highlight: Hierarchical Tree vs Flat Action List

The spec proposes flat Vector[MigrationAction] where each action carries its path:

// Spec approach
case class DynamicMigration(actions: Vector[MigrationAction])
case class AddField(at: DynamicOptic, default: SchemaExpr[?]) extends MigrationAction

Per JDG feedback

“nested migrations can’t be flat, must be lists of lists where top level encodes depth”

, this implementation uses a recursive tree:

// This implementation
sealed trait MigrationStep {
  case class Record(
    fieldActions: Vector[FieldAction],      // Actions at this level
    nestedFields: Map[String, MigrationStep] // Recursive children
  )
}
sealed trait FieldAction {
  case class Add(name: String, defaultValue: DynamicValue)  // No path, just name
}

This enables compile-time validation of deeply nested paths:

case class PersonV1(name: String, address: Address)
case class PersonV2(name: String, address: AddressV2)
case class AddressV2(street: String, city: String, zip: String) // nested field added

// .build validates that address.zip is handled, not just top-level fields
MigrationBuilder.from[PersonV1].to[PersonV2]
  .add(_.address.zip, DynamicValue.Primitive("00000"))
  .build  // Compile error if nested field missing

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