Feat/cart loyalty post order

[omsaisudarshan108]

Summary

• Add Cart capability schema, REST endpoints, and spec docs for multi-item baskets.
• Add Loyalty checkout extension with discovery/linking flow and examples.
• Add Post-Order Management extension for returns/exchanges/refunds on Order.
• Regenerate spec outputs and update mkdocs navigation.

Testing

• python validate_specs.py

Additional Notes

• Schemas introduced under source/schemas/shopping/ with generated spec/ updates.

[jingyli]

Thank you for your contribution to the Universal Commerce Protocol!

While we appreciate your enthusiasm for UCP, this doesn't align with our process for Enhancement proposals. We encourage you to explore either submitting a formal proposal for TC review or implementing these as vendor specific capabilities/extensions and have them live & hosted on your own domain so they can be referenced in the profile as long as they follow UCP's governance model.