Add routed_expert_logits support to xorl-client R3 datums

[kiddyboots216]

Problem

The server-side R3 path can accept routed_expert_logits, but xorl-client cannot currently express it.

Today:

• Datum supports routed_experts but not routed_expert_logits.
• TrainingClient.forward_backward(...) only serializes fields present on each Datum.

That leaves a real SDK gap and forces the docs to under-document or mis-document the feature.

Acceptance criteria

• Add optional routed_expert_logits support to Datum.
• Plumb it through datum serialization / conversion in TrainingClient.
• Add tests covering serialization of both routing fields.