The metadata mirror is padded to kNPadded (65536) but only primitiveCount
slots are live. metadataBuffer.FlushDevice() wgpuWriteBuffer'd all 256 KB
through the WASM->JS staging path every frame (~100-250x waste for a
few-hundred-instance scene). Switch to the existing FlushDeviceRange
overload — the same one the instanceBuffer loop directly above uses —
sized to primitiveCount * sizeof(uint32_t). The Vulkan parallel already
sizes its flush to primitiveCount, so this was WebGPU-specific.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
