Raise the threshold for minimum buffer size from 1 GiB to 4 GiB, based on real-world experiments of overcommitting device memory with model weights larger than available VRAM, for example Qwen3.5-35B-A3B-Q8 running on a B70. Also add a debug message to better track USM system allocations. Signed-off-by: Francois Dugast <francois.dugast@intel.com>