Replace per-instance cuBLAS/cublasLt handle ownership in CublasForward
with Arc<SharedCublasHandle>. The struct is renamed to CublasGemmSet
and a type alias preserves backward compatibility for external callers.
Removed from CublasGemmSet: handle, workspace, lt_handle, lt_workspace,
bias kernels (all now in SharedCublasHandle). Kept: branch streams,
branch workspaces, GEMM caches, events, network dimensions.
External callers (gpu_dqn_trainer, gpu_experience_collector) will be
updated in Tasks 4-5 to pass Arc<SharedCublasHandle> to the constructor.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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