Implements a pure, algebraic migration system for ZIO Schema that represents
structural transformations between schema versions as first-class, serializable data. Unlike traditional migration approaches, this system uses no functions, closures, or reflection - migrations are pure ADTs that can be stored, inspected, and applied dynamically.

Key Features

• Pure Data Migrations: All migrations are represented as case classes with no closures
• Fully Serializable: DynamicMigration can be stored in registries, databases, or files
• Compile-Time Validation: MigrationBuilder.withFieldTracking ensures all
  fields are handled
• Bidirectional: Every migration has a structural reverse
• Path-Based Actions: All operations specify location via DynamicOptic
• Introspectable: Migration logic can be inspected, transformed, and optimized
• Tree-Structured Nesting: Nested migrations are recursive action trees
• Cross-Branch Field Operations: Join/split fields across different document branches

API Overview

// Build a type-safe migration
val migration = MigrationBuilder.withFieldTracking[PersonV1, PersonV2]
  .renameField(select(_.name), select(_.fullName))
  .keepField(select(_.age))
  .addField(select(_.country), "US")
  .build  // Only compiles when all fields are handled

// Apply migration
migration.apply(v1) // Right(PersonV2(...))

// Reverse migration
migration.reverse.apply(v2) // Right(PersonV1(...))

// Nested migrations
  val migration = MigrationBuilder.withFieldTracking[PersonV0, PersonV1]
    .keepField(select(_.name))
    .inField(select(_.address), select(_.address))(addressMigration)
    .build

Migration Actions Category: Record Actions: AddField, DropField, Rename, TransformValue, Mandate, Optionalize,
ChangeType ──────────────────────────────────────── Category: Enum Actions: RenameCase, TransformCase ──────────────────────────────────────── Category: Collection
Actions: TransformElements, TransformKeys, TransformValues ────────────────────────────────────────
Category: Nested Actions: TransformField, TransformEachElement, TransformEachMapValue

• Core Types: MigrationError, MigrationAction, DynamicMigration, Migration,
  SchemaFields, FieldSelector, Resolved, PrimitiveConversions, MigrationSchemas
• Type-Level: FieldSet (Scala 2 & 3)
• Macros: SelectorMacros, SchemaFieldsMacros, MigrationBuilderMacros (Scala 2 &

• Builder: MigrationBuilder (Scala 2 & 3)
• Documentation: docs/reference/migration.md

Design Decisions

1. Non-inline builder methods: Only select() and withFieldTracking are inline
   macros. All builder methods are regular methods with type parameters, ensuring builders work correctly when stored in vals.
2. Two-layer architecture:

• DynamicMigration - Untyped, fully serializable core
• Migration[A, B] - Typed wrapper with schemas

3. Resolved expressions: Pure data representations of value computations
   (Literal, Identity, FieldAccess, Convert, Concat, etc.)

Test Coverage

24 spec files covering:

• Core migration operations
• Builder API and field tracking
• All migration action types
• Bidirectional migrations
• Error handling and diagnostics
• Algebraic properties (identity, associativity)
• Edge cases and boundaries
• Property-based tests

fixes #519 /claim #519