Student terminals often render Unicode math fine; ask before assuming

I had a saved rule that a student's terminal couldn't show Unicode math, so I rendered equations to PNGs when they asked for LaTeX. They corrected me: Unicode symbols like omega, pi, sigma and subscripts work fine in a modern terminal such as Ghostty, and it keeps them in the chat. Default to Unicode math in code blocks, and keep PNGs for plots. Note that subscript letters are limited in Unicode, so use x_d style when a letter has no subscript form.