fix: preserve directory size on read_dir failure and strengthen FUSE mount probe

When read_dir() fails, return Info with the already-computed size instead
of Info::default(), which was discarding the directory's own inode size.

Also tighten the FUSE mount readiness check to require Some(Ok(_)) so
the probe only succeeds when a directory entry can actually be read.

https://claude.ai/code/session_01LfpnUZrgq93MVZgA3KVqE6

Author: claude

src/fs_tree_builder.rs

@@ -140,7 +140,10 @@ where
                                 path,
                                 error,
                             }));
-                            return Info::default();
+                            return Info {
+                                size,
+                                children: Vec::new(),
+                            };
                         }
                         Ok(entries) => entries,
                     }

tests/one_file_system.rs

@@ -230,7 +230,7 @@ fn cross_device_excludes_mount() {
         .map(Duration::from_millis)
         .map(sleep)
         .filter_map(|()| mount_point.read_dir().ok())
-        .find_map(|mut entry| entry.next());
+        .find_map(|mut entry| entry.next()?.ok());
     assert!(
         poll_result.is_some(),
         "FUSE mount at {mount_point:?} not ready after {retries} retries"