fix(fuse): skip unreadable folders in sync, never evict local-only files

An unreadable subfolder, such as a container's root-owned cache, failed
every pass of its synced folder, so nothing else in the tree reached
Drive (#33, B196). The scan now leaves such a path out, together with an
entry that vanished mid-scan and a name that is not UTF-8, and the pass
drops everything under it from classification, the wipe guard and the
push pass's deletions. It is never read as deleted and never downloaded
into. An unreadable or absent root still fails the pass. The activity
feed names unreadable paths once each time the set changes.

Two related bugs on the way to online-only:

- The push pass read deletions from the raw baseline, so a synced path
  that later became ignored was trashed on Drive (B197).
- The switch evicted everything once the folder was idle, including
  ignored paths such as .git/, symlinks, unreadable folders and files
  changed after the pass, none of which are on Drive (B198). It now
  refuses and names the path.

Closes #33

crates/pdfs-fuse/src/sync/model.rs

@@ -98,6 +98,11 @@ pub(super) struct Outcome {
     pub(super) errors: usize,
     /// Files skipped because another process held them open for writing.
     pub(super) deferred: usize,
+    /// Paths the local scan could not read, left out of the pass. Not part of
+    /// [`is_empty`](Self::is_empty) or the summary: an unreadable folder stays
+    /// unreadable from one poll to the next, so the pass reports it once, when
+    /// the set changes, rather than on every poll.
+    pub(super) unscanned: Vec<String>,
 }
 
 impl Outcome {