segfaults at threefs tests

[takeshi-yoshimura]

reported in #91

[gitbisector]

get_device() returns cuda:0 whenever a GPU is present, so submit_io allocates device memory and MockFileReader.read_chunked does ctypes.memmove into it — a host copy through a device pointer. The mock already documents that dev_ptr must be host memory; nothing enforced it. CI has no GPU runners, hence green there.

Same tree on a GB10:

--gpus all  ->  exit 139 (SIGSEGV)
no GPU      ->  4 passed

It kills the interpreter, so pytest tests/unit can't complete on a GPU host either. PR shortly: pick a GPU only when the real reader is installed.