docs: add compute module README

compute/ is the recommended dispatch surface for backend-agnostic GPU computing but has no README. All other GPU-related modules (vcl, cuda, vulkan) have READMEs.

Task

Create compute/README.md explaining:

1. The compute abstraction layer
2. How it dispatches to vcl/cuda/vulkan backends
3. Available operations (element-wise, BLAS, reduce)
4. Basic usage example

Verification

• README links correctly to backend-specific docs