Runtime and SIMD dispatch

Every kernel dispatches at runtime to the best available SIMD path; the dispatch implementation is private.

onnx-light-cpu owns SIMD computation, not thread scheduling. Direct C++ kernel calls execute synchronously on the calling thread and do not create workers. When the kernels are registered with onnx-light, the registration adapter injects the session CpuExecutor. Large ranges are then split into disjoint SIMD-aligned blocks and dispatched by that executor.

Consequently, participant count, affinity, spin policy, nesting, lifecycle, and diagnostics all come from the onnx-light session policy. There are no ONNX_LIGHT_CPU_NUM_THREADS or ONNX_LIGHT_CPU_SPIN_COUNT settings and no second pool that can oversubscribe the runtime.