#988 highway_dsl WorkflowBuilder.build() should reject branch handlers defined outside the branch lambda

EARS: When WorkflowBuilder.build() runs, it shall raise ValueError if any task inside a ParallelOperator/MultiChoiceOperator branch (recursively) has on_failure_task_id/on_success_task_id not in that branch's tasks, with the same message as the engine's BranchHandlerScopeError (#966). Engine already rejects at submit; this moves the error to authoring time. Repo ~/develop/highway_dsl (modifications allowed per memory highway_dsl); requires a release and uv.lock bump.