fix partial download progress showing 0% on restart

On restart, _emit_existing_download_progress() checked
downloaded_bytes_this_session to decide if a download was pending.
Since this field is always 0 in a new session, partially downloaded
models were reported as DownloadPending (0%) instead of DownloadOngoing
with their actual progress. Check downloaded_bytes (actual data on
disk) instead.

Closes #1042

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/exo/download/coordinator.py

@@ -324,7 +324,7 @@ class DownloadCoordinator:
                                 shard_metadata=progress.shard,
                                 total_bytes=progress.total_bytes,
                             )
-                        elif progress.downloaded_bytes_this_session.in_bytes == 0:
+                        elif progress.downloaded_bytes.in_bytes == 0:
                             status = DownloadPending(
                                 node_id=self.node_id,
                                 shard_metadata=progress.shard,