api(quanticstci): re-export AbstractTensorTrain for tensor_train() consumers

[shinaoka]

Problem

QuanticsTensorCI2::tensor_train() returns a TensorTrain<V>, but to do anything useful with it (evaluate, sum), the user needs AbstractTensorTrain trait from tensor4all-simplett. This trait is not re-exported from tensor4all-quanticstci, so downstream crates must add tensor4all-simplett as an explicit dependency.

Found by

Feature test report: docs/test-reports/test-feature-20260327-quanticstci.md

Suggestion

Re-export AbstractTensorTrain from tensor4all-quanticstci (or at minimum document that tensor4all-simplett is needed as a dependency to use the returned TT).

Severity

Low — ergonomics improvement