Summary

This PR implements a pure, algebraic migration system for ZIO Schema that represents structural transformations between schema versions as first-class, serializable data.

Key Features

• Pure Data Migrations: All migrations are represented as case classes with no closures, making them fully serializable
• Hierarchical Actions: Supports nested migrations via AtField, AtCase, AtElements, enabling migrations like _.address.street -> _.address.streetName
• Bidirectional: Every migration has a structural reverse
• Introspectable: Migration logic can be inspected, transformed, and used to generate DDL, upgraders, or downgraders

Architecture

• DynamicMigration: Untyped, fully serializable core that operates on DynamicValue
• Migration[A, B]: Typed wrapper with schemas for compile-time safety
• MigrationAction: Hierarchical ADT with “lists of lists” structure for nested migrations (the key innovation per @jdegoes feedback)
• ResolvedExpr: Pure expressions for value-level transformations (no closures)
• MigrationBuilder: Fluent API for constructing migrations

Example Usage

import zio.blocks.schema.migration._

case class PersonV1(firstName: String, lastName: String)
case class PersonV2(fullName: String, age: Int)

val migration = MigrationBuilder[PersonV1, PersonV2]
  .joinFields(Vector("firstName", "lastName"), "fullName",
    ResolvedExpr.concatWith(" ")(
      ResolvedExpr.field("firstName"),
      ResolvedExpr.field("lastName")
    ))
  .addFieldInt("age", 0)
  .build

// Nested migrations
val nestedMigration = MigrationBuilder[WithAddressV1, WithAddressV2]
  .atField("address")(
    _.renameField("street", "streetName")
     .addFieldString("zipCode", "00000")
  )
  .build

Test plan

• All 33 migration tests pass on Scala 3
• All 33 migration tests pass on Scala 2.13
• Existing schema tests still pass
• CI passes

/attempt #519 /claim #519