feat: async GPU upload pipeline and asset streaming

[Hekbas]

Implement asynchronous asset streaming with background GPU upload to avoid frame hitches when loading large textures and meshes.

Tasks

• Create a staging buffer ring for async GPU uploads (separate transfer queue)
• Implement a priority queue for pending asset uploads
• Stream textures: upload lowest mip first, progressively upload higher mips
• Stream meshes: support LOD-based progressive loading
• Add placeholder resources (pink texture, unit cube) while assets stream in
• Budget system: limit upload bandwidth per frame to avoid GPU stalls
• Integrate with AssetManager load paths
• Monitor and display streaming status in the profiler