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/CHANGELOG.md

@@ -70,9 +70,10 @@ Schema: **44**, unchanged.
 
 - **A synced folder whose files were all gone from this computer no longer gets stuck** (#46).
   The sync still refuses to delete everything on Proton Drive, but a switch to online only now
-  goes ahead and shows what Proton Drive has, instead of waiting for good. To download the
-  files again instead, sync the folder from scratch. Restoring folders also logs why it
-  skipped a folder.
+  goes ahead and shows what Proton Drive has, instead of waiting for good. This includes a
+  switch asked for right after the files went, which was refused with "… was deleted here but
+  not on Proton Drive yet". To download the files again instead, sync the folder from scratch.
+  Restoring folders also logs why it skipped a folder.
 
 - **The first sync of a folder shows how far its scan got** (#46). Before anything downloads,
   the first pass reads every item on Proton Drive, which takes many minutes for a large folder.