Automatically verify images exist on disk in final `verification_and_fallback_matching.py` script

> Add verification step to ensure all images in the mapping file actually exist on disk, that would detect all of these problems

Refer to https://github.com/07th-mod/hou-plus-og-sprites-new/issues/6 for more details

Look at the script `verification_and_fallback_matching.py` to understand its current mapping file format and image loading logic. Add a step that reads the mapping file, checks each referenced image path exists on disk, and logs or handles missing files. Run the script on sample data to verify the new check works.

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