[treeform]

Summary

• expose gl_InstanceID as Shady's portable instance identifier
• map it to gl_InstanceID for desktop/Web GLSL, gl_InstanceIndex
  for Vulkan GLSL, SV_InstanceID for HLSL, and [[instance_id]]
  for Metal
• map Vulkan's vertex identifier to gl_VertexIndex
• allow local shader object declarations without emitting invalid synthetic
  constructors
• cover portable identifiers and local parameter structs in backend codegen
  tests
• compile the Metal instance-ID path with Apple's native shader compiler

Why

Stateless GPU particle systems and other instanced vertex shaders need a
per-instance identifier without uploading a CPU-generated ID buffer. Every
supported graphics API provides this concept, but each backend uses a
different builtin spelling or semantic.

Impact

Shader authors can use gl_InstanceID once in Shady source and generate valid
desktop GLSL, WebGL 2 GLSL, Vulkan GLSL, HLSL, and Metal output. Complex
instanced shaders can also assemble and pass flat local parameter structs
without Shady inventing an unsupported default constructor.

Root cause

Shady exposed gl_VertexID but had no shared instance-ID builtin or target
mappings. Vulkan also inherited OpenGL's vertex builtin name, while HLSL and
Metal entry generation did not inject their native instance semantics.

Validation

• nim r tests/test_backends.nim
• nim r tests/test_shady.nim
• nim r tests/test_cpu.nim
• nim r -d:shadyRunMetal tests/test_metal.nim