detokenization

Decode each token on its own and an emoji streams as two replacement characters. The answer builds the offset-window decoder that fixes it, runs it, and adds the stop-string holdback that most streaming bugs come from.

The stop string arrives as three tokens, the euro sign arrives as two, and the client has already been sent half of both. The answer holds back exactly the text that could still become a stop, decodes bytes incrementally, and tests the stream against a reference that sees the whole output at once.