fuse: reset treeCache on snapshot reload to fix stale latest symlink (#21873)

The treeCache in SnapshotsDir was never cleared when snapshots were
reloaded. This caused the "latest" symlink to keep pointing to the
previous snapshot even after new snapshots were added.

Add a generation counter to SnapshotsDirStructure that is incremented
whenever the directory structure is rebuilt (in makeDirs). The
treeCache checks this generation on each lookup and resets itself
when the generation changes, ensuring cached nodes (including symlinks)
are refreshed after a snapshot reload.

internal/fuse/dir.go

@@ -225,7 +225,7 @@ func (d *dir) Lookup(ctx context.Context, name string) (fs.Node, error) {
 		return nil, err
 	}
 
-	return d.cache.lookupOrCreate(name, func(forget forgetFn) (fs.Node, error) {
+	return d.cache.lookupOrCreate(name, -1, func(forget forgetFn) (fs.Node, error) {
 		node, ok := d.items[name]
 		if !ok {
 			debug.Log("  Lookup(%v) -> not found", name)