Document pcPrimitiveIndex fragment shader built-in

[mvaligursky]

TO BE MERGED TO MAIN WHEN ENGINE 2.15 is out

Documents the new pcPrimitiveIndex fragment shader built-in variable in the WGSL specifics page:

• Add primitiveIndex to FragmentInput built-ins list
• Add pcPrimitiveIndex to global scope names
• Add note explaining WebGPU-only availability, device capability check, and CAPS_PRIMITIVE_INDEX shader define

[willeastcott]

When updating the User Manual, ask Cursor to also translate the Japanese docs in i18n. We probably need an AGENTS.md to hint that.

[mvaligursky]

Ok. That will need to be a separate PR, as there are multiple changes in these files in few PRs