Document VuMark instance image differences from real Vuforia

[adamtheturtle]

Summary

• Adds a "VuMark instance images" section to differences-to-vws.rst noting that the mock returns a fixed minimal PNG image and does not encode the instance_id into the image (unlike real Vuforia)

This is extracted from the docs portion of #2954, which is applicable regardless of that PR's other changes.

🤖 Generated with Claude Code

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Note

Low Risk
Low risk documentation-only change that does not affect runtime behavior.

Overview
Documents a known discrepancy for VuMark instance image responses: the mock returns a fixed minimal image in the requested format and does not encode the requested instance_id, unlike real Vuforia.

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