fix(fuse): let an emptied mirror folder switch to on-demand at once

A switch asked for before any pass saw the folder empty found it still
idle, so the check for what only this computer has ran at once and
refused it for good: every synced path read as "deleted here but not on
Proton Drive yet". The account run hit it in the B255 case.

A local copy gone in full is the wipe guard's case, which the push pass
of a queued switch settles without trashing anything on Drive. The
switch now returns NotNow for it, so the request is queued and that pass
runs first. A partial deletion is still refused as before.

docs/BUGS.md

@@ -402,14 +402,20 @@ folder lost its files is not known yet.
 
 **Fix.** The push pass for a switch to on-demand forgets the baseline rows of a local copy that
 is gone in full, instead of trashing them on Drive, so the switch goes ahead and the mount shows
-what Drive has. **Sync From Scratch…** in the app and `pdfs sync reset ID` clear a mirror
+what Drive has. A switch asked for before any pass saw the folder empty found it still `idle`,
+and the check for what only this computer has refused it at once: every path read as "deleted
+here but not on Proton Drive yet". The account run found that. Such a switch now waits for the
+same push pass (`mirror_tree_gone` in `crates/pdfs-fuse/src/sync.rs`, checked in
+`apply_sync_folder_mode`). **Sync From Scratch…** in the app and `pdfs sync reset ID` clear a mirror
 folder's baseline, so the next pass compares both sides as on the first sync: one-sided files
 are copied across, files that differ keep both versions, and nothing is deleted. Restore now
 logs why it skipped a folder.
 
 **Test:** The simulations `a_folder_whose_local_copy_is_gone_still_goes_on_demand` (which
-failed in exactly the reported state before the fix) and
-`a_folder_whose_local_copy_is_gone_downloads_again_after_a_reset`.
+failed in exactly the reported state before the fix),
+`a_folder_emptied_before_any_pass_still_goes_on_demand` (which failed with the account run's
+error) and `a_folder_whose_local_copy_is_gone_downloads_again_after_a_reset`; the acceptance case
+"an emptied mirror folder switches to on-demand".
 
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