Foundation encodes `encode(_:forKey: "UID")` as a `$top` dictionary
whose single key is `UID`. The keyed archive projection treated every
one-key `UID`/`CF$UID` dictionary as a reference marker, so that root
became a malformed reference and the real link to its object was lost.
Treat a one-key dictionary as a UID only when its value is a number,
matching the binary decoder's UID shape and Foundation's XML `CF$UID`
rule; otherwise descend into it as an ordinary dictionary.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
