#2491 fix(setup): don't require tensor-data alignment for GGUF shards without …

说明

## Change

Skip the tensor-payload extent check when a successfully parsed GGUF shard has no tensors. GGUF parsing, the existing checks for tensor-bearing shards, and the separate pinned size/SHA-256 verification remain unchanged.

`check_shards()` and `whole_shard()` calculated the minimum GGUF shard size as `data_start + max(tensor end offsets)`. For a shard containing no tensors, this incorrectly required the file to extend through the aligned tensor-data start, even though no tensor payload exists.

This caused valid metadata-only GGUF shards to be rejected as truncated. For example, `Qwen3.8-Flash-Next-UD-Q5_K_XL-00001-of-00006.gguf` was reported as six bytes short:

- Actual file size: 10,946,618 bytes
- Parsed tensor count: 0
- Aligned data start: 10,946,624 bytes

The downloaded file matched the size and SHA-256 declared in `setup.py`.


## Title
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Issue: Resolves #2470 and #2346  

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## What changed
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