hf-fetch: only pass -C - when a non-empty partial exists

With a missing file, -C - sends Range: bytes=0-, which HF's xethub
CDN rejects with 400. Xet-backed repos (e.g. Lucebox/DeepSeek-V4.1-
Flash-ROCMFP23-GGUF) fail on the first fetch; the old LFS backend
tolerated the zero-offset range.

scripts/hf-fetch.sh

@@ -62,8 +62,12 @@ fetch_one() {
         echo "  refetch $name (have $cur_size, want $size)"
     fi
 
-    # resumable download; -f fails on HTTP error so a 401/403 is not written as a file
-    curl -fsSL -C - -K "$auth_file" -o "$path" "$url"
+    # Resumable download; -f fails on HTTP error so a 401/403 is not written as a file.
+    # -C - only when a non-empty partial exists: with a missing file it sends
+    # Range: bytes=0-, which HF's xethub CDN rejects with 400 (xet-backed repos).
+    local resume_args=()
+    [[ -f "$path" && -s "$path" ]] && resume_args=(-C -)
+    curl -fsSL "${resume_args[@]}" -K "$auth_file" -o "$path" "$url"
 
     # verify byte count
     local got