Match Python cache fallback when symlinks are unavailable

[i386]

Summary

Mirror Python huggingface_hub cache behavior for platforms where snapshot symlinks cannot be created, especially Windows without Developer Mode/admin symlink privileges.

• Try to create the snapshot pointer as a symlink on Windows instead of unconditionally copying.
• If symlink creation fails, move a freshly downloaded blob into the snapshot path so a new download does not leave both blobs/<etag> and snapshots/<commit>/<filename> as full-size files.
• Continue copying existing blobs into snapshots when symlinks are unavailable, because an existing blob may already be referenced by other snapshots.
• Reuse an existing regular snapshot file after metadata lookup when the cache is already in degraded no-symlink mode and the blob path is absent.

Python reference

This follows the Python huggingface_hub cache fallback behavior:

• Symlink support is probed per cache directory and Windows users without symlink support are warned that cache behavior is degraded: https://github.com/huggingface/huggingface_hub/blob/07a6ccc819fd531c3fdb984954133c27271a412a/src/huggingface_hub/file_download.py#L2354-L2452
• _create_symlink documents the degraded behavior: newly downloaded blobs are moved into the destination, while existing blobs are copied to avoid breaking other snapshots: https://github.com/huggingface/huggingface_hub/blob/07a6ccc819fd531c3fdb984954133c27271a412a/src/huggingface_hub/file_download.py#L608-L634
• The actual fallback branch moves when new_blob=True and copies otherwise: https://github.com/huggingface/huggingface_hub/blob/07a6ccc819fd531c3fdb984954133c27271a412a/src/huggingface_hub/file_download.py#L693-L699

Why

The Python implementation avoids keeping both the extensionless blob and the named snapshot file for a fresh download when symlinks are unavailable. The current Rust implementation always copies on Windows, which can double disk usage for large model files.

Tests

Added focused regression coverage for the degraded no-symlink path:

• pointer_fallback_moves_new_blob verifies fresh blobs are moved to the snapshot path and the blob path is removed.
• pointer_fallback_copies_existing_blob verifies existing blobs are copied so other snapshot references remain valid.
• cache_download_reuses_regular_snapshot_when_blob_is_absent verifies a cache already in degraded no-symlink mode reuses the regular snapshot file instead of downloading again.

Validation

• cargo fmt --check with nightly rustfmt
• cargo test -p hf-hub --lib (167 passed)
• cargo clippy -p hf-hub --all-features -- -D warnings
• git diff --check