This PR marks `levelZero` and `Level.ofNat` as `@[implicit_reducible]`
so that `Level.ofNat 0 =?= Level.zero` succeeds when the definitional
equality checker respects transparency annotations. Without this,
coercions between structures with implicit `Level` parameters fail, as
reported by @FLDutchmann on
[Zulip](https://leanprover.zulipchat.com/#narrow/channel/113488-general/topic/backward.2EisDefEq.2ErespectTransparency/near/576131374).
🤖 Prepared with Claude Code
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
d59f229b74