feat: add per-seller offer attribution to catalog variants

[proshoumma]

Adds an optional offers[] array to the catalog variant, so one catalog can list a product sold by several different merchants and send each to the right seller's checkout.

Description

Today the business serving a catalog is assumed to be the only seller and Merchant of Record (MoR): variant.id is the checkout item ID and variant.price is the price checkout honors. That doesn't fit a marketplace or shopping agent that lists the same product from many merchants, each with its own checkout. offers[] fixes that. A variant without offers[] is unchanged.

Each offer names the seller (seller.domain, the MoR), that seller's own checkout item ID (item_id), and the price. This affects the catalog capabilities dev.ucp.shopping.catalog.search and dev.ucp.shopping.catalog.lookup, which share the variant type.

New type source/schemas/shopping/types/offer.json — required: seller (with seller.domain), item_id, price. Optional: list_price, unit_price, availability, url, sku, seller.name, seller.links.

{
  "id": "gid://shop/variant/123",
  "title": "Wireless Headphones",
  "price": { "amount": "19.99", "currency": "USD" },
  "offers": [
    { "seller": { "domain": "shop-a.example" }, "item_id": "A-item-1", "price": { "amount": "19.99", "currency": "USD" } },
    { "seller": { "domain": "shop-b.example" }, "item_id": "B-item-9", "price": { "amount": "21.50", "currency": "USD" } }
  ]
}

When offers is present (Business response → Platform), the Platform:

• checks out with the chosen offers[].item_id — MUST NOT use variant.id
• treats offers[].price as the real price — variant.price is display-only
• resolves offers[].seller.domain and checks out at that seller, after verifying it via authority binding

Also aligns the Business / MoR wording in glossary.md and core-concepts.md so a business need not be the MoR for everything it lists.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

• [Proposal]: Add per-seller offers to catalog variants #810 – Enhancement Proposal for this change.
• Domain Working Group (DWG) Charter: Bulk Product Discovery #682 – Bulk Product Discovery DWG — this fills the seller-attribution slice its charter defers)
• feat: define namespace authority binding + broaden RDNS grammar #530 – namespace authority bindingthis reuses for seller.domain
• [Proposal]: Add Provider participant type and offer discovery to dev.ucp.common.identity_linking #384 – this is an alternative to participant-actor approach

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

Not applicable.