fix(core,fuse): never stat a foreign mount from the local-file indexer

The "This computer" indexer walks $HOME, and `default_excludes` only kept it
out of the Drive mountpoint and our own state/cache dirs. A home directory
that also holds third-party FUSE mounts (sshfs, rclone) got walked into.

`scan` does ask for `same_file_system(true)`, which looks sufficient and is
not: `ignore` implements it as `is_same_file_system` -> `device_num(path)` ->
`path.metadata()`, run after the directory is queued. Learning that a mount is
foreign requires stat'ing it first, and against a mount whose server is gone
that stat never returns.

The consequence is worse than a stalled scan. The blocked thread sits in
uninterruptible FUSE wait, so it ignores SIGKILL: the daemon can never exit,
systemd's stop times out, and the unreapable process keeps the cache.db lock
the next start needs, turning one dead mount into a permanent restart loop.
No userspace recovery exists once a thread is in that state, so the fix has to
be to never enter it.

Exclude every mountpoint strictly below the scan root, read by path from
/proc/self/mounts (octal-unescaped, so a path with a space still matches).
`filter_entry` runs before the device stat, so the walker never touches them.
Re-read per scan rather than cached at startup: a mount that appeared since
the daemon started is the likeliest to be half-alive.

This does not change what is indexed — a nested mount is a different device,
so it was already going to be skipped. Only the stat in front of that decision
goes away.

Refs: docs/BUGS.md B90

crates/pdfs-fuse/src/background.rs

@@ -487,7 +487,12 @@ fn scan_local_once(
         }
     };
 
-    let excludes = localindex::default_excludes(mountpoint, &dirs.state_dir(), &dirs.cache_dir());
+    let mut excludes =
+        localindex::default_excludes(mountpoint, &dirs.state_dir(), &dirs.cache_dir());
+    // Resolved per scan rather than once at startup: mounts come and go, and a
+    // mount that appeared since the daemon started is exactly the one most
+    // likely to be a half-alive network filesystem (see `nested_mount_points`).
+    excludes.extend(localindex::nested_mount_points(&home));
     indexing.store(true, Ordering::Relaxed);
     let started = Instant::now();