Override shader chunks

Overriding a StandardMaterial chunk keeps fog, tone mapping, shadows, skinning, and instancing intact. Use a ShaderMaterial only when the look needs its own vertex and fragment pair.

• Set material.shaderChunksVersion to the installed major.minor, such as '2.23'. Chunk names and contracts change between versions, so read the installed chunk source, not memory.
• Set chunks with material.getShaderChunks(SHADERLANGUAGE_GLSL).set(name, source) and the WGSL equivalent, then call material.update(). For every material on a device, use ShaderChunks.get(device, language); the global shaderChunks object is deprecated.
• Supply both GLSL and WGSL. On WebGPU, one GLSL-only chunk sends the whole shader through a transpiler that is usually not loaded, so it fails to compile. Otherwise pin the device to WebGL.
• Clone shared materials, including imported ones, before overriding them for only some meshes. Per-mesh uniforms use meshInstance.setParameter().
• Uniforms are raw; convert sRGB colours with Color#linear().
• Vertex chunks that read the model matrix, such as transformCoreVS, must keep the INSTANCING path with a matrix_model fallback, like the stock chunk.
• Expand mesh bounds by the maximum displacement; CPU bounds do not see shader deformation.
• For an emissive-only look, implement getEmission() writing dEmission, zero diffuse and specular, and disable useSkybox.