subtype: Treat free TypeVars as singleton identities (#61876)

This PR updates the semantics of free TypeVars in subtyping to act as
egality-compared atoms (as proposed in
https://github.com/JuliaLang/julia/issues/61242#issuecomment-4008886548).
Previously free typevars
in subtyping were undefined behavior although they generally behaved
universally (unless the same typevar by identity was present in a
UnionAll). This fixes #61242 by making sure that `Vector !=
Vector.body`.

Written by GPT 5.5.

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Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
Co-authored-by: Codex <codex@openai.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>