fix(shadertools): repair WebGPU glTF material shader compilation

[ibgreen-openai]

Goals

• Restore real WebGPU rendering for glTF assets that combine textured base-color and transmission materials.
• Make shader-module binding allocation stable across independently specialized material permutations.
• Preserve valid WGSL derivative and uniform-control-flow semantics for diffuse-transmission image-based lighting.

Changes

• Keep automatically assigned module bindings globally unique within each WebGPU bind group, including bindings registered by previously compiled but currently inactive shader permutations.
• Preserve stable existing assignments, explicit/application bindings, cross-group isolation, and paired texture/sampler allocations.
• Sample diffuse irradiance cubemaps with explicit mip level zero in both shared-scene and standalone glTF environment paths, eliminating implicit derivatives inside data-dependent transmission branches.
• Add regression coverage for transmission-only, base-color-only, and combined shader permutations.
• Add WGSL source and available-device compilation regressions for textured transmission with both environment configurations.

Root causes

1. The persistent WGSL binding registry allocated the same group-3 binding to separate optional texture bindings when those bindings first appeared in disjoint shader permutations; a later combined material then failed during binding reservation.
2. Diffuse-transmission IBL sampled an environment cubemap with implicit derivatives after branching on a texture-derived factor, which violates WGSL uniform control-flow requirements.

Verification

• 36 focused shader-assembly and PBR regression tests pass.
• Strict cross-package ANARI/glTF/shadertools TypeScript project-reference build passes.
• All four changed files pass Biome formatting and git diff checks.
• Verified directly in the live WebGPU Animation Studio: Physical Transmission renders successfully, switching to the 86-joint animated robot and back succeeds, and no application shader errors are emitted.

Scope

• Four shadertools files only.
• Based directly on current master; no unpublished showcase, resource-lifecycle, or animation-instance changes.

[chatgpt-codex-connector]

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