[pathfinder] Use `LOAD_WITH_ALTERED_SEARCH_PATH` for system DLL search on Windows

[rwgk]

WIP WIP WIP — Bug discovered while testing suggested code for NVIDIA/numba-cuda#308

This fix was generated by Cursor. I still need to review it myself.

The bug went unnoticed because on Windows we always have CUDA_HOME or CUDA_PATH set in our testing.

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When loading CUDA DLLs via system search on Windows, the previous approach using LoadLibraryExW with flags=0 would find the DLL on PATH but fail to locate its co-located dependencies (error 126).

This fix uses SearchPathW to first find the DLL's full path, then loads it with LOAD_WITH_ALTERED_SEARCH_PATH so Windows searches for dependencies starting from the DLL's directory.