Part of #188474.

The public compute surface.

Changes

• ComputePipeline built from a single compute Shader, mirroring RenderPipeline.
• CommandBuffer.createComputePass returning a ComputePass with bind methods for storage buffers, uniforms, and textures, plus dispatch(x, y, z).
• Wire to ComputePassMTL.
• Add a basic supportsCompute getter so apps can query support. The loud tier error and fallback guidance are tracked separately.

Testing

A Dart test that runs a known compute kernel on Metal and verifies the output.

Dependencies

#188478, #188479.