compute shader: moving physics to the GPU

- entity position updates now run on GPU via compute shader
- GPU-based RNG for entity velocity randomization
- full simulation loop stays on GPU, no CPU roundtrip
- new compute.zig module for shader management
- GpuEntity struct with position, velocity, and color
- tracy profiling integration
- FPS display turns green (good) or red (bad)
- added design docs for zoom/pan and compute shader work
- cross-platform alignment fixes for shader data