[Feat] ONNX export/import

[ax1s-x1zz]

Context

docs/ROADMAP.md Track D-2. Exported models unlock ecosystem interop and serving.

Scope

• TrainedModel → ONNX export
• ONNX → inference without re-training

Acceptance

Exported ONNX runs in onnxruntime with the same prediction as xazz predict.

Dependencies

Depends on D-1 (device mapping).

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See docs/ROADMAP.md for the full scale plan and execution order.