content: move Nathanael Jacquier to alumni

[chengjiahao1234]

Type of Change

• Content Update (news, publications, team)

Summary

Move Nathanaël Jacquier to Alumni following completion of his research internship.

Changes Made

• _data/internships.yml: remove his active internship entry.
• _data/alumni_members.yml: record his 2026 research internship and current academic affiliation.
• Preserve his photo, email, and LinkedIn metadata without displaying personal links.

Testing Checklist

• Desktop (1920x1080), tablet (768x1024), and mobile (375x667)
• Light and dark themes
• Appears once in Alumni and no longer in Internships
• No broken images, console errors, or horizontal overflow
• Ran bundle exec jekyll serve successfully
• YAML validation and git diff --check

Additional Notes

• Affiliation: engineering student at CentraleSupélec; admitted to the MVA Master's program at ENS Paris-Saclay.
• Based on his publicly indexed LinkedIn announcement; direct profile access was blocked.
• No degree completion is implied. Other team entries remain unchanged.
• Screenshots captured locally; not attached. No related issue.

By submitting this PR, I confirm:

• Tested locally and followed DEVELOPMENT.md
• Documentation changes are unnecessary for this content-only update
• Ready for review