feat(media): model image, video, and 3D variants

[igrigorik]

The existing Media shape is image-oriented: one URL and one pair of dimensions cannot faithfully describe alternate video resolutions and codecs, adaptive-streaming manifests, device-specific 3D formats, poster images, or an externally hosted player. This update extends Media as an open type discriminator with well-known image, video, and model_3d variants. It follows the schema.org MediaObject family while preserving UCP's existing wire contract and generated-source APIs.

• Adds per-type schemas for image, video, and 3D media using open if/then composition.
• Adds sources for the complete set of available video renditions or 3D encodings.
• Adds preview for a poster or thumbnail still.
• Adds optional name, distinct from accessibility-oriented alt_text.
• Adds video duration in seconds.
• Models externally hosted playback as type: "video" plus embed_url, rather than introducing a redundant external_video type.
• Adds source metadata for MIME type, format, dimensions, and byte size.
• Keeps Media.type open: a Platform must ignore media types it does not recognize.

When sources is present and url identifies a rendition, the Business should include a source whose url matches the media item's url. The exception is an externally hosted video, where url identifies a watch page rather than a playable rendition.

Self-hosted video

The primary rendition appears in both url and the complete sources set:

{
  "type": "video",
  "url": "https://cdn.example.com/video-720.mp4",
  "name": "Product demonstration",
  "duration": 24,
  "preview": {
    "url": "https://cdn.example.com/video-poster.jpg",
    "width": 1280,
    "height": 720
  },
  "sources": [
    {
      "url": "https://cdn.example.com/video-720.mp4",
      "mime_type": "video/mp4",
      "width": 1280,
      "height": 720
    },
    {
      "url": "https://cdn.example.com/video.m3u8",
      "mime_type": "application/vnd.apple.mpegurl"
    }
  ]
}

3D model

A single logical model can advertise the formats supported by different devices:

{
  "type": "model_3d",
  "url": "https://cdn.example.com/product.glb",
  "preview": {
    "url": "https://cdn.example.com/product-poster.jpg"
  },
  "sources": [
    {
      "url": "https://cdn.example.com/product.glb",
      "mime_type": "model/gltf-binary"
    },
    {
      "url": "https://cdn.example.com/product.usdz",
      "mime_type": "model/vnd.usdz+zip"
    }
  ]
}

Externally hosted video

The watch page remains the primary URL while embed_url identifies third-party active player content:

{
  "type": "video",
  "url": "https://videos.example.com/watch/123",
  "embed_url": "https://videos.example.com/embed/123",
  "preview": {
    "url": "https://cdn.example.com/video-poster.jpg",
    "width": 1280,
    "height": 720
  }
}

A Platform may decline to load third-party active content under its security, privacy, or presentation policy and use the watch-page URL instead.

Backward compatibility

This is backward compatible for conforming uses of the released Media contract:

• the required set remains type and url;
• no existing field is removed, renamed, or retyped;
• width and height remain on base Media for generated-source compatibility;
• all new fields are optional;
• existing payloads continue to validate;
• unknown media types remain valid.

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Checklist

• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities.
• I have followed the Contributing Guide
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.