Add model cards and artifact-integrity checks

[unes07]

Problem

Packaged model artifacts need clear intended-use boundaries and automated checks that their registry metadata, feature order, dependencies, and binary contents remain consistent.

Proposed work

• Add a model card for every published crop model.
• Document training data, features, targets, preprocessing, intended and prohibited uses, geographic coverage, metrics, uncertainty, and known limitations.
• Record package versions, artifact hashes, and registry schema version.
• Add CI tests that load every registry entry, verify feature order, run inference, and compare metadata with the artifact.

Acceptance criteria

• Every registry entry links to a complete model card.
• Artifact hashes and required library versions are verified automatically.
• Inference smoke tests cover every packaged model.
• Registry features and targets match the loaded artifact.
• Cards report spatial-validation results and dataset versions rather than only headline random-split metrics.

Depends on #2, #3, and #4.