Add labels to split view sandcastles (like Gaussian Splats)

[jjspace]

Initially noticed for the new Gaussian Splatting Sandcastle which doesn't make it clear which side is the splats and which is the 3d tiles mesh. If you're a dev or familiar with splats it may be "obvious" but not everyone will be. We should add labels to sandcastles that have a split so it's clear what each side is.

We have a handful of Sandcastles that use the split feature but only one of them labels what each side is showing