Function GetPartIndexVaryingDeclaration

Returns an include-guarded declaration of the vPartIndex varying shared by the Gaussian Splatting material plugins (debug, solid-color, GPU picking). More than one of these plugins can be attached to the same material at once; each needs vPartIndex to carry splat.partIndex from the vertex to the fragment stage. Without a guard each plugin injects its own varying vPartIndex, which is a duplicate declaration — fatal under WGSL ("redefinition of 'vPartIndex'"). The guard makes the declaration idempotent so exactly one survives regardless of how many plugins inject it. Inject the result at a CUSTOM_VERTEX_DEFINITIONS / CUSTOM_FRAGMENT_DEFINITIONS point.