Validate checkpoint contracts before constructing xScan models

[tpn]

The xScan device pipeline and stage benchmark checked checkpoint dimensions and feature identity after constructing, transferring, and optionally compiling the model. An incompatible checkpoint could therefore incur expensive setup before reporting the mismatch.

Let the loader run the existing contract validator immediately after checkpoint deserialization, and use it in both consumers. Existing policy, model, device, and file-identity checks remain in place. This change does not limit checkpoint deserialization or valid model sizes.

Validation: 106 focused tests passed; 50 optional GPU tests skipped. The regression exercises the real loader for both consumers and rejects incompatible image size and feature identity before model construction. All repository pre-commit checks passed.

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📒 Files selected for processing: 5

• src/cuphoton/xscan/device_pipeline.py
• src/cuphoton/xscan/pipeline_benchmark/stages.py
• src/cuphoton/xscan/training.py
• tests/xscan/test_device_pipeline.py
• tests/xscan/test_tensor_inference.py

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📝 Walkthrough

The model loader now accepts an optional checkpoint validator and calls it before model construction. Worker initialization and benchmark setup pass checkpoint-contract validation to the loader. Tests cover validation order and callback behavior.

Changes: Checkpoint validation

Layer / File(s)	Summary
Add a loader validation hook src/cuphoton/xscan/training.py, tests/xscan/test_tensor_inference.py	load_model_from_checkpoint accepts an optional validator and invokes it on the loaded checkpoint before building the model. Tests check that the callback receives the returned checkpoint once.
Wire validation into worker and benchmark setup src/cuphoton/xscan/device_pipeline.py, src/cuphoton/xscan/pipeline_benchmark/stages.py, tests/xscan/test_device_pipeline.py	Worker initialization and benchmark setup pass checkpoint-contract validation to the loader. Tests check that image-size and feature-identity mismatches raise before model construction.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 05641

Checkpoint contract checks now run before model construction in both setup paths. No actionable issue remains from this review.

Pre-merge checks: 2

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[melo-gonzo]

LGTM. Checkpoint contracts now fail before model construction while preserving existing loader behavior. Focused CPU tests, rejection probes, and both selected GPU pipeline tests passed. Deserialization still precedes validation.