This PR enables `backward.whnf.reducibleClassField` for v4.29.
The support is particularly important when the user marks a class field
as `[reducible]` and
the transparency mode is `.reducible`. For example, suppose `e` is `a ≤
b` where `a b : Nat`,
and `LE.le` is marked as `[reducible]`. Simply unfolding `LE.le` would
give `instLENat.1 a b`,
which would be stuck because `instLENat` has transparency
`[instance_reducible]`. To avoid this, when we unfold
a `[reducible]` class field, we also unfold the associated projection
`instLENat.1` using
`.instances` reducibility, ultimately returning `Nat.le a b`.
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Co-authored-by: Paul Reichert <6992158+datokrat@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Kim Morrison <kim@tqft.net>
93683eb455