cudarc's stream.alloc_zeros uses cuMemAllocAsync (stream-ordered).
These allocations are only accessible from the allocating stream.
When cublasLtMatmul runs on a forked BRANCH stream (multi-stream
branch dispatch), the stream-ordered workspace is inaccessible,
causing CUBLAS_STATUS_NOT_SUPPORTED on H100 at batch=4096.
Fix: allocate workspace via cuMemAlloc_v2 (synchronous, globally
accessible from all streams). This matches the C++ debug test
which uses cudaMalloc and works on H100 for all dimensions.
Also: add d_layout destroy (leaked handle cleanup).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
06f9819f05