MKVNOTE csv tagging: tags are case sensitive

See title vs TITLE in screenshot below:

I added a title column is csv in lower case, and ran `./mkvnote test.csv` 

<img width="1096" height="765" alt="Image" src="https://github.com/user-attachments/assets/6bc246c1-7428-436a-b20f-a558ab66aeca" />
