AWS Batch (batch) restJson1 service for fakecloud.
Batch 0 (foundation): vendored Smithy model + crate scaffold + restJson1
routing for all 45 operations + the core control plane (compute
environments, job queues, job definitions, tags). Real container-backed job
execution (SubmitJob -> ECS task) and the remaining resource families land
in later batches; unimplemented operations return a faithful
ServerException/not-implemented error rather than a fake success.