fix(fuse): stop a sync pass when nearly all of a large folder vanishes

The wipe guards refused a pass only when every synced path was missing
on one side. One surviving file cleared them, so a half-mounted disk,
a restored backup that brought back one file, or a script that emptied
the folder and wrote one back made the next pass trash everything else
on Drive, and on every computer syncing the folder (B270).

Both guards now also refuse when nine in ten or more of the tracked
paths of a folder with at least 20 are missing at once. A smaller
folder still stops only on a total loss, since an ordinary cleanup can
delete most of it. The reason now gives the count.

B270 stays partly fixed: a smaller loss still goes through, and the
app does not yet offer to confirm the deletions.

docs/TROUBLESHOOTING.md

@@ -248,9 +248,10 @@ Check that:
 
 ### A synced folder stays in error and nothing downloads
 
-The log says `every one of the N synced paths is missing locally; refusing to trash them on
-Drive`. The folder synced before, and now none of its files are on this computer. The sync
-stops so that it does not delete them on Proton Drive too. Check that the folder is the right
+The log says `M of the N synced paths are missing locally; refusing to trash them on Drive`.
+The folder synced before, and now all or nearly all of its files are gone from this computer:
+everything, or nine in ten of a folder with 20 or more files and folders. The sync stops so that
+it does not delete them on Proton Drive too. Check that the folder is the right
 one and that its disk is mounted. To get the files back, choose **Sync From Scratch…** in the
 folder's menu on **Sync → Folders**, or run: