Summary

This PR implements a pure algebraic migration system for ZIO Schema 2 as requested in #519.

Features Implemented

• MigrationAction sealed trait - 5 reversible actions: AddField, DropField, Rename, SetValue, Optionalize
• DynamicMigration - Pure data structure with no closures, fully serializable
• Migration[A, B] - Type-safe wrapper with compile-time schema enforcement
• Path-based navigation via DynamicOptic for nested structures
• Bidirectional migrations - All migrations are reversible
• Composition via ++ operator with identity and associativity laws

Testing

• 10 new tests covering all migration operations
• All 2180 tests pass (2170 existing + 10 new)
• Demo script included at schema/jvm/src/test/scala/zio/blocks/schema/migration/MigrationDemo.scala

Demo

Run the demo:

sbt "schemaJVM/testOnly *MigrationSpec*"

Or run the full test suite:

sbt schemaJVM/test

Claim

/claim #519

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