This PR adds the ability to register theorems with the `cbv_eval`
attribute in the reverse direction using the `←` modifier, mirroring the
existing `simp` attribute behavior. When `@[cbv_eval ←]` is used, the
equation `lhs = rhs` is inverted to `rhs = lhs`, allowing `cbv` to
rewrite occurrences of `rhs` to `lhs`.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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