[BUG-FIX]: download progress not reported to Python during file transfers

[tobocop2]

This blocks the companion PR huggingface/huggingface_hub#4059

Related issue: huggingface/huggingface_hub#4058

Problem

Download progress bars in huggingface_hub update in large jumps (e.g. 0% -> 26% -> 63% -> 100% for a 4GB file) instead of smoothly. The root cause is that ItemBridgeState::compute_diff hardcodes all total_transfer_bytes fields to 0, so the fine-grained per-HTTP-chunk progress that xet-core already tracks internally never reaches Python callbacks.

Fix

• Add transfer_bytes and transfer_bytes_completed to ItemProgressReport
• Update ItemBridgeState::compute_diff to diff transfer fields, matching the existing GroupBridgeState behavior
• Update GroupBridgeState::compute_diff item inclusion to also trigger on transfer progress

Reproduction

Single file

"""Verify smooth single-file download progress."""
import tempfile
from huggingface_hub import hf_hub_download

with tempfile.TemporaryDirectory() as tmp:
    hf_hub_download(
        "Qwen/Qwen3-4B",
        filename="model-00002-of-00003.safetensors",
        cache_dir=tmp,
        force_download=True,
    )

Multi-file

"""Verify smooth multi-file download progress."""
import tempfile
from huggingface_hub import snapshot_download

with tempfile.TemporaryDirectory() as tmp:
    snapshot_download(
        "Qwen/Qwen3-4B",
        allow_patterns=["model-00003-of-00003.safetensors", "config.json", "*.txt"],
        cache_dir=tmp,
        force_download=True,
    )

Before: tqdm bar jumps 0% -> 26% -> 63% -> 100% with ~4 callbacks for a 4GB file.
After: tqdm bar updates smoothly every 250ms.

See callback_bridge.rs tests for coverage of the new transfer byte diffing.

[hoytak]

LGTM -- makes sense.