Extract simplego backend into standalone module

[ajroetker]

Summary

• Extracts the simplego backend from gomlx/gomlx and go-highway contrib packages from ajroetker/go-highway into a standalone github.com/gomlx/backend module
• Inlines the highway SIMD bridge directly into the simplego package, removing the interface/registration indirection (HighwayMatMul interface, RegisterHighway(), side-effect import pattern)
• Consolidates two worker pool implementations (internal/workerspool mutex-based pool and pkg/workerpool channel-based pool) into a single pkg/workerpool package
• Adds go tool directives for hwygen and goat with the ajroetker/goat fork replace

Module layout

gomlx/backend/
├── go.mod              # module github.com/gomlx/backend
├── gomlx/              # simplego backend (package simplego) with inlined highway SIMD
├── pkg/
│   ├── matmul/         # from go-highway contrib/matmul
│   ├── nn/             # from go-highway contrib/nn
│   ├── activation/     # from go-highway contrib/activation
│   ├── packgemm/       # packed GEMM implementation
│   └── workerpool/     # consolidated worker pool
└── internal/
    └── cmd/            # code generators (simplego_generator, simplego_dispatcher,
                        #   alternates_generator, packgemm_generator)

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./gomlx/... passes
• go test ./pkg/... passes (matmul, nn, activation, packgemm, workerpool)
• go tool hwygen --help resolves correctly
• go tool goat --help resolves correctly