[bitsandbytes] Warning "No prebuilt binary for CUDA X.Y, loading CUDA A.B instead" / BNB_CUDA_VERSION override silently pinning a wrong library

Cause (Documented platform behavior): Selection: exact match, else highest lower same-major, else lowest higher same-major; no cross-major fallback. Overrides bypass selection entirely with no fallback. Fix status: documented_behavior Other error fragments: - value must be digits only (e.g. '124' for 12.4). Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/bitsandbytes-foundation/bitsandbytes/833649043474794b8fe7a4136e0c40faf077b2e0/bitsandbytes/cextension.py (official_docs, unknown, documented_behavior): get_cuda_bnb_library_path docstring and warnings describe selection, override, and digits-only validation. - https://raw.githubusercontent.com/bitsandbytes-foundation/bitsandbytes/833649043474794b8fe7a4136e0c40faf077b2e0/docs/source/errors.mdx (official_docs, unknown, documented_behavior): "Library not found: version mismatch" section documents automatic fallback and the override variable. Search phrasings: bitsandbytes BNB_CUDA_VERSION environment variable detected; bitsandbytes No prebuilt binary for CUDA loading instead Evidence basis (self-declared by the contributing chat client): public_source.