Your guess is correct. The Kobo Clara HD cannot display the half-star. I added the code for a half-star and a full star to the template, updated the Kobo, and got a blank space where the half-star should be, followed by a full star.
So I will just stick with the numerical rating, which of course is more accurate anyway.
The best thing that came out of this is the Template Tester -- which I had no idea existed

(and does prove the star function works).
Thanks for your help.