A refused key and an unavailable fresh key are now the model's answer:
ReplaceKey returns the kept key with the notice text, FreshKey returns
null with its notice, and IMGUI only shows them.
A key is taken when another serializes the same or when the dictionary's
own comparer calls them equal, so 0.0 against -0.0 is refused instead of
throwing in BuildDictionary.
Keys never actually compared by bytes: the Studio handed the model the
document codec, whose registry guard throws for any non-document value,
so every key fell back to ToString. CultCacheMessagePack now builds the
model with a value serializer, and the Studio and tests both use it.
Tests pin the comparer rule, the unchanged-key rule (an old empty
reference key survives an edit of its value) and serialized identity
(keys whose ToString collides); each fails with its rule removed.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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