Add EXT_meshopt_compression support

[relh]

Summary

• decode ATTRIBUTES, TRIANGLES, and INDICES meshopt streams in pure Nim
• apply NONE, OCTAHEDRAL, QUATERNION, and EXPONENTIAL filters
• resolve compressed buffer views before existing accessor reads, including required-extension placeholder buffers

Proof

• nim check src/gltf/meshopt.nim
• nim check src/gltf/reader.nim
• nim r -d:release tests/test_meshopt.nim
• nim r -d:release tests/test_mesh_quantization.nim
• CI sparsely checks out Khronos glTF Sample Assets at 9429648735279342b4c32b8745f7904196607379
• CI loads both AnimatedMorphCube/glTF-Quantized and BrainStem/glTF-Meshopt-EXT, asserting their required extensions, renderable geometry, and supported status
• local BrainStem/glTF-Meshopt-EXT result: 49 primitives and 34,084 decoded points
• official meshoptimizer-derived byte vectors cover all three modes and all four filters
• loaded real compact Hades assets including static desert_bones.glb, multi-mesh inf_forest_flower_00.glb, and skinned/animated wolf.glb, glub.glb, donkey.glb, and bunny.glb
• git diff --check

Stacked on #50 because compact meshopt glTFs also use KHR_mesh_quantization. The decoder implementation follows meshoptimizer under its MIT license; no JavaScript or native binary dependency is added.