consensus: changelog for Nary.inject changes

ouroboros-consensus/changelog.d/20250219_054827_nick.frisby_simplify_inject.md

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+### Patch
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+### Non-Breaking
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+### Breaking
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+- Make the type of `Nary.inject` more precise.
+  The old type involves oracular data, so any required changes downstream are almost certainly limited to testing code.
+  (I'm therefore tempted to list this as Patch instead --- violating the [PVP](https://pvp.haskell.org/) --- but the risk-reward seems prohibitive.)