Rename COPYING to LICENSE for GitHub license detection

[arijitroy003]

Summary

• Renames COPYING to LICENSE so GitHub can automatically detect and display the project's license (BSD 3-Clause for code, CC-BY-SA 4.0 for content)
• File contents are unchanged; this is a filename-only change

Verification

• Confirmed COPYING exists and LICENSE does not prior to this change
• Confirmed no references to the COPYING filename exist in README.md or other project files
• After rename, GitHub will show the license badge on the repository page