Fix/pose rotation

[DuinoDu]

Summary

Add pose visualization tooling and documentation, plus correct quaternion component ordering in pose loading.

Details

• Add examples/python/vis_pose.py to render RGB + pose cube visualizations and encode an output MP4 via ffmpeg.
• Document pose axis conventions and link the sample visualization in README.md.
• Include sample output video docs/vis_pose_result.mp4.
• Fix pose quaternion mapping in src/spatialmp4/reader.cc to read x/y/z/w in the correct order.
• Update .gitignore to ignore video/*.mp4 while keeping video/test.mp4 tracked.