The steps to merge were counted from the undo history's length, which
stops growing at its 1000-step limit: with a full history nothing merged,
or only the last part was relabelled "Synchronize Folder" while the
import stayed behind it. Session now counts committed steps
(Session::commits) and Synchronize merges by that count.
Once the import is in, a later part that fails is listed in `failed`
instead of aborting with the import left as a separate step.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>