Fixes #519 /claim #519

Implements a pure algebraic schema migration system for ZIO Schema 2.

What this adds

• typed Migration[A, B] built on serializable DynamicMigration
• path-based migration actions over DynamicValue
• migration builder support for record, enum, collection, and map transformations
• structural reverse support and composition laws
• Scala 3 selector macro support for building DynamicOptic paths
• Scala 2 compatibility path for migration builder usage
• migration law, runtime, and macro-focused test coverage

Implementation notes

• fixes Scala 2 migration macro compilation by importing DynamicOptic in the Scala 2 macro implementation
• adds missing license headers in version-specific migration sources/tests
• formats the migration sources/tests so repo lint passes cleanly

Local verification

• sbt "++2.13.18" "schemaJVM/testOnly zio.blocks.schema.migration.DynamicMigrationSpec zio.blocks.schema.migration.MigrationSpec zio.blocks.schema.migration.MigrationLawSpec"
• sbt "++3.3.7" "schemaJVM/testOnly zio.blocks.schema.migration.DynamicMigrationSpec zio.blocks.schema.migration.MigrationSpec zio.blocks.schema.migration.MigrationLawSpec zio.blocks.schema.migration.MigrationMacroSpec"
• sbt "++3.3.7" "schemaJS/testOnly zio.blocks.schema.migration.DynamicMigrationSpec zio.blocks.schema.migration.MigrationSpec zio.blocks.schema.migration.MigrationLawSpec zio.blocks.schema.migration.MigrationMacroSpec"
• sbt "++2.13; check; headerCheckAll; ++3.7; check; headerCheckAll"

This branch is based directly on main and can be reviewed independently.