Fixes: #32738

Description

##Problem
Currently, instanceIndex is used for two different purposes:

• In vertex shaders: mesh instance index (WebGPU instance_index)
• In compute shaders: global invocation index (computed from globalId)

This causes confusion because:

• The term "instance" typically refers to geometry instancing, not compute invocations
• It conflicts with WebGPU/Vulkan terminology, which uses "invocation" for compute shaders
• The dual meaning makes code harder to understand and maintain
• Documentation is unclear about when to use which meaning

##Solution
Introduced a new invocationIndex constant specifically for compute shader invocations, while keeping instanceIndex for mesh instances only. This aligns with WebGPU/Vulkan terminology and clarifies the distinction.