Implement a pure, algebraic, fully serializable migration system for ZIO Schema 2 as described in issue #519. The system follows the dual-layer architecture of Patch/DynamicPatch: an untyped DynamicMigration core with a typed Migration[A, B] wrapper.

Key components:

• MigrationAction: 14 sealed action types with structural reverse
• MigrationExpr: 10 pure expression types (zero closures)
• MigrationError: 8 path-aware error variants via DynamicOptic
• DynamicMigration: Full interpreter with navigation, arithmetic, type coercion, and option unwrapping
• Migration[A, B]: Typed wrapper with apply, compose, reverse
• MigrationBuilder: Fluent builder with build-time validation

Verified: 197 tests pass on Scala 2.13.18, 3.3.7, and 3.7.4. Uses Chunk throughout to match codebase conventions. /claim #519