feat(gltf): launch Animation Studio and curated expressive assets

[ibgreen-openai]

Goals

• Make the new glTF animation, skinning, morph-target, camera, and variant capabilities visible in a real user-facing studio.
• Provide a carefully licensed gallery of expressive, reproducible glTF assets.
• Let users independently animate several copies of the same imported character on both supported GPU backends.

Changes

• Vendor the official CC0 RobotExpressive.glb (463,988 bytes): 14 named animation clips, two 43-joint skeletons, and three facial morph targets.
• Build a complete Animation Studio with named clips, playback speed, seeking, looping, crossfades, facial-expression sliders, authored cameras, material variants, and up to six independently animated characters.
• Add a curated CC0 asset manifest spanning skins, morph targets, instancing, visibility, transmission, iridescence, sheen, diffuse transmission, and experimental volume scattering.
• Surface RobotExpressive, SimpleSkin, and AnimatedMorphCube in the ANARI playground, and expose its existing binary GLB export capability.
• Export the canonical glTF morph-weight API and expand the public showcase documentation.
• Label candidate and experimental material extensions explicitly; preserve all upstream license and provenance information.

Verification

• Real native Chromium WebGL and WebGPU: 32 browser integration tests across four suites.
• Node, RobotExpressive import, retained-scene animation, and restored provenance: 29 tests across seven suites.
• Strict standalone showcase TypeScript and all transitive ANARI/glTF package references pass.
• Full production Docusaurus build, raw-document generation, standalone glTF and ANARI production builds pass.
• Actual production website loads the 86-joint robot and exposes every animation control with no browser console errors.
• Independently based on the repaired current master; no unrelated provenance files appear in the PR diff.

Companion work

• Independent animation-instance runtime: feat(gltf): support independently animated scenegraph instances #2991.
• Shared glTF resource caching and explicit asset ownership: feat(gltf): reference-grade asset lifecycle, shared resources, and complete API docs #2992.
• Experimental physical material extensions: companion PR forthcoming.