fs: add Data field to Tree for user-attached state

Enables session trees to carry *Session without a separate map.
Part of killing Manager — tree IS the session collection.

fs/tree.go

@@ -25,6 +25,7 @@ type Tree struct {
 	statFn    func([]string, string) (os.FileInfo, error)
 	readdirFn func([]string, string) ([]os.DirEntry, error)
 	children  map[string]*Tree
+	Data any // user-attached state
 }
 
 // Option configures a Tree.