[WIP] Fix docs example for MgVideo average method issue

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Docs example uses mv.average() but MgVideo has no average method</issue_title>
<issue_description>- Reproduce issue #325 using the code snippet from docs/examples.md (Example 8).

• Determine whether:
  • average() was renamed/removed (update docs to correct method), or
  • it should exist on MgVideo (implement/restore and add tests).
• Acceptance criteria:
  • Example 8 in docs/examples.md works as written or is updated to the correct API.
  • Add a small regression test (or minimal smoke test) covering the documented behavior.</issue_description>

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