Two changes for GGUF variant selection:
1. Default variant preference now starts with UD-Q4_K_XL (Unsloth
Dynamic quantization) which provides better quality per bit than
standard Q4_K_M. Also added UD-Q2_K_XL, UD-IQ2_M, UD-IQ1_M,
UD-IQ1_S as small fallback options.
2. If the selected variant doesn't fit on disk, automatically fall
back to the smallest GGUF variant in the repo that does fit.
Queries all GGUF file sizes via get_paths_info() and picks the
smallest one under the free disk space limit. If nothing fits,
raises a clear error.
This means users with limited disk space won't get a download
error -- they'll get a smaller quantization instead.
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