Add KHR_mesh_quantization support

[relh]

Summary

• decode quantized mesh and morph-target accessors through the canonical Nim reader
• apply signed and unsigned glTF normalization with component-sized strides
• validate extension-specific attribute layouts and cover padded VEC3 data

Proof

• nim check src/gltf/reader.nim
• nim r -d:release tests/test_mesh_quantization.nim
• CI sparsely checks out Khronos glTF Sample Assets at 9429648735279342b4c32b8745f7904196607379
• CI loads AnimatedMorphCube/glTF-Quantized and asserts that it requires KHR_mesh_quantization, produces renderable geometry, and is not reported unsupported
• local sample result: 1 primitive and 24 decoded points
• loaded real quantized Hades horse.glb, knight.glb, and skeleton_mage.glb; all meshes, skins, and animations parsed with no unsupported extensions
• git diff --check

The focused sample test imports the reader directly. The repository-wide nim check src/gltf.nim remains sensitive to the locally installed Shady version; the CI dependency environment owns integrated coverage.