Alternator

Endpoint-local adapter for fallback output shapes.

The endpoint AST stores one abstraction that can be backed by either zio.blocks.combinators.Eithers on all Scala versions or zio.blocks.combinators.Unions on Scala 3. This keeps HttpCodec.Fallback and auth/error composition platform-agnostic while still delegating all combine / separate logic to the combinators block library.